[Feature Request] [Enhancement] Systemd service file

### Summary:
The [systemd service unit](https://github.com/evilsocket/opensnitch/blob/master/daemon/opensnitchd.service) can be enhanced to start **BEFORE** any network is configured, and thus allow interception and protection at an earlier stage...

### Service file:
Below is my crafted service file:
```ini
# /etc/systemd/system/opensnitchd.service
[Unit]
Description=Application firewall OpenSnitch
Documentation=https://github.com/gustavo-iniguez-goya/opensnitch/wiki
Documentation=man:systemd.special
Documentation=man:systemd.service
Documentation=man:systemd.exec
Documentation=man:systemd.unit

DefaultDependencies=no
Before=network-pre.target shutdown.target
Wants=network-pre.target
Conflicts=shutdown.target
# Don't start when 'no-appfw` is in kernel command-line, to allow booting without it.
ConditionKernelCommandLine=!no-appfw

[Service]
Type=exec
ConfigurationDirectory=%N/rules
ConfigurationDirectoryMode=0700

Environment='custom_cfg=%E/%N/rules'
# Environment='opts=-debug'

ExecCondition=%N -check-requirements
ExecStart=%N -rules-path $custom_cfg $opts

# Signal-info was taken from the init.d script, but it just exits and then systemd restarts the service...
ExecReload=kill -HUP $MAINPID
Restart=always
RestartSec=30
TimeoutStopSec=10
# Ensure it is not killed by the Linux kernel's Out-Of-Memory (OOM) killer.
# https://www.freedesktop.org/software/systemd/man/systemd.exec.html#OOMScoreAdjust=
OOMScoreAdjust=-1000

[Install]
WantedBy=basic.target
```

---
Notable changes/additions:

1. **Unit filename:**
:warning: The service filename was changed to `opensnitchd.service` !!!
This was needed for automatically using the unit name in the service definition using the `%N` specifier...
See: [Specifiers](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Specifiers)@man `systemd.unit#Specifiers`
- _The previous filename was lacking the `d` in it's name !_
- Thus if you want to use this service file while **at same time** having the _currently distributed_ unit file in your system you need to "mask" the old one to prevent it from being used and started:
```bash
sudo systemctl mask --now opensnitch.service
```

1. **Unit ordering:**
Made sure the daemon starts before any network related devices or services are created/started by using `network-pre.target`, See:
- [Network pre-configuration](https://systemd.io/NETWORK_ONLINE/)@systemd.io
- [how to start a systemd service before networking starts?](https://unix.stackexchange.com/a/354607/550161)@unix.stackexchange

1. **Startup target / run-level:**
Changed the default install target to `basic.target` instead of `multi-user.target`.
This will allow it to run in any "run-level" in SysV terms.
See: [Units managed by the system service manager](https://www.freedesktop.org/software/systemd/man/systemd.special.html#basic.target)@man `systemd.special#basic.target`

1. **Automatically disable using a kernel-command-line option:**
Just in case it is needed, i added the ability to disable the daemon using a kernel-command-line option using the `ConditionKernelCommandLine` directive.
When the `no-appfw` option is present in the kernel-command-line, the daemon will not startup.
This option functions same as the well-known `quite` option. _(only applies when present as a separate word)_

1. **Automatically create rules directory:**
Let systemd automatically create the "rules" directory with proper mode, when non-existent yet, upon starting the daemon service by using the `ConfigurationDirectory` and `ConfigurationDirectoryMode` directives.

1. **Automatically check for kernel support before starting:**
Automatically prevent startup when required kernel support is not present by using the `-check-requirements` flag in a `ExecCondition` directive.
_(This assumes the command returns a **non-zero exit status** when not satisfied.)_

1. **Reload functionality:**
Added support for reloading the daemon using the `ExecReload` directive.
Signal-info was taken from the init.d script, but it just exits and then systemd restarts the service...
So this functionality either needs to be implemented in the daemon's code or a different signal needs to be sent to it.
But at least the functionality is now present in the unit file.

1. **Prevent from being killed by the OOM-Killer:**
Prevented the daemon to be killed by the Linux kernel's Out-Of-Memory (OOM) killer, using the `OOMScoreAdjust=-1000` directive.
See: [OOMScoreAdjust](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#OOMScoreAdjust=)@man `systemd.exec#OOMScoreAdjust`
_(This will ensure that the protection keeps functioning even when other processes cause an OOM)_

1. **Admin overrides using drop-ins:**
Added support for easily adjusting the directory used for rules and extra options by the local admin.
The local admin can create "drop-in" config(s) under `/etc/systemd/system/opensnitchd.service.d/` even when the service file is installed in other places by the package maintainer, See: [16.14. Extending the default unit configuration](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/assembly_working-with-systemd-unit-files_configuring-basic-system-settings#proc_extending-the-default-unit-configuration_assembly_working-with-systemd-unit-files)@redhat.com
Example drop-in(s) contents that _**can**-be-used_:
- To change the rules directory to be used:
```ini
[Service]
Environment='custom_cfg=%E/%N/rules-special'
```
- The contents of `$custom_cfg` is supplied as an argument to the `-rules-path` option of the daemon.
- See: [Specifiers](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Specifiers)@man `systemd.unit#Specifiers`
`%E` expands to `/etc`
`%N` expands to the unit name `opensnitchd`
- Thus the total expansion of the above will become: `/etc/opensnitchd/rules-special`

- To enable debug output:
```ini
[Service]
Environment='opts=-debug'
```
- The contents of `$opts` is supplied as extra argument(s) to the daemon.

- Combination of both:
```ini
[Service]
Environment='custom_cfg=%E/%N/rules-special'
Environment='opts=-debug'
```

---
One last thing to improve systemd usage:
Please add a way to **prevent timestamps being output at all**, when using `/dev/stdout` as log destination.
Or at least add a `syslog` option to use as logger, which also doesn't need timestamps because it adds them self.
Look how ugly it is at moment:
```term
$ systemctl status opensnitchd

● opensnitchd.service - Application firewall OpenSnitch
Loaded: loaded (/etc/systemd/system/opensnitchd.service; enabled; preset: enabled)
Active: active (running) since Tue 2023-08-15 16:09:57 +03; 1h 47min ago
Docs: https://github.com/gustavo-iniguez-goya/opensnitch/wiki
man:systemd.special
man:systemd.service
man:systemd.exec
man:systemd.unit
Process: 593 ExecCondition=opensnitchd -check-requirements (code=exited, status=0/SUCCESS)
Main PID: 632 (opensnitchd)
Tasks: 26 (limit: 38131)
Memory: 81.4M
CPU: 1min 42.501s
CGroup: /system.slice/opensnitchd.service
└─632 opensnitchd -rules-path /etc/opensnitchd/rules

Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 13:09:57] IMP Starting opensnitch-daemon v1.6.1
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 13:09:57] INF Loading rules from /etc/opensnitchd/rules ...
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 13:09:57] INF loading domains lists: lists, lists.domains_regexp, /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-allow
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 13:09:57] INF monitor lists started: /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-allow
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 13:09:57] INF loading domains lists: lists, lists.domains_regexp, /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-reject
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 13:09:57] INF monitor lists started: /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-reject
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 16:09:57] INF [eBPF] module loaded: /usr/lib/opensnitchd/ebpf/opensnitch.o
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 16:09:57] INF [eBPF] module loaded: /usr/lib/opensnitchd/ebpf/opensnitch-procs.o
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 16:09:57] INF Process monitor method ebpf
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 16:09:57] INF Stats, max events: 25, max stats: 150, max workers: 6
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 16:09:57] INF nftables config changed, reloading
Aug 15 16:09:57 kubuntu opensnitchd[632]: [2023-08-15 16:09:57] INF fw configuration loaded
Aug 15 16:09:58 kubuntu opensnitchd[632]: [2023-08-15 16:09:58] INF Using nftables firewall
Aug 15 16:09:58 kubuntu opensnitchd[632]: [2023-08-15 16:09:58] INF Running on netfilter queue #0 ...
Aug 15 16:09:58 kubuntu opensnitchd[632]: [2023-08-15 16:09:58] INF [eBPF] module loaded: /usr/lib/opensnitchd/ebpf/opensnitch-dns.o
Aug 15 16:09:58 kubuntu opensnitchd[632]: found /lib/x86_64-linux-gnu/libc.so.6
Aug 15 16:10:03 kubuntu opensnitchd[632]: [2023-08-15 16:10:03] INF clearing domains lists: 0 - /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-reject
Aug 15 16:10:03 kubuntu opensnitchd[632]: [2023-08-15 16:10:03] INF clearing domains lists: 0 - /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-allow
Aug 15 16:10:03 kubuntu opensnitchd[632]: [2023-08-15 16:10:03] INF 6 regexps loaded, /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-reject/re.txt
Aug 15 16:10:03 kubuntu opensnitchd[632]: [2023-08-15 16:10:03] INF 1 lists loaded, 6 domains, 0 duplicated
Aug 15 16:10:03 kubuntu opensnitchd[632]: [2023-08-15 16:10:03] INF 19 regexps loaded, /mnt/LutrisGames/OpenSnitch-Firewall-Lists/WoT-Blitz-allow/re.txt
Aug 15 16:10:03 kubuntu opensnitchd[632]: [2023-08-15 16:10:03] INF 1 lists loaded, 19 domains, 0 duplicated
Aug 15 16:10:35 kubuntu opensnitchd[632]: [2023-08-15 16:10:35] INF Connected to the UI service on ///tmp/osui.sock
Aug 15 16:10:35 kubuntu opensnitchd[632]: [2023-08-15 16:10:35] IMP UI connected, dispathing queued alerts: 0
Aug 15 16:10:35 kubuntu opensnitchd[632]: [2023-08-15 16:10:35] INF Start receiving notifications
```