add more flags to systemd generate command

In order to better customize the generated systemd service, fwdctl should provide more flags to change default parameters.
An example could be:
```
generates systemd service file to run fwdctl at boot

Usage:
fwdctl generate systemd [flags]

Flags:
-f, --file string rules file (default "rules.yml")
-h, --help help for systemd
-p, --installation-path string fwdctl installation path (default "/usr/local/bin")
-t, --type string systemd service type [oneshot, fork] (default "oneshot")
-s, --restart-sec systemd service restart seconds (works only with service of type "fork") (default: 5s)

Global Flags:
-O, --output-path string output path
```
Where a new parameter (`-s`) is provided and works only with service type `fork`.