Faulty error detection, leads to crash: "[fatal] This is the first launch of AdGuard Home. You must run it as Administrator."

Issue Details
---

### Context

* **Version of AdGuard Home server:**
* v0.107.7
* **How did you install AdGuard Home:**
* `docker pull adguard/adguardhome:latest`
* docker version: 20.10.12
* **How did you setup DNS configuration:**
* N/A
* **If it's a router or IoT, please write device model:**
* N/A
* **CPU architecture:**
* amd64
* **Operating system and version:**
* Ubuntu Server LTS 20.04
* **Result of `./AdGuardHome -v --version`:**
* See at the end of this bug report

### Description

Under docker, when running AdGuardHome as a non-root user, AdGuardHome displays the following and exits
```
2022/07/06 15:46:36.407393 [info] AdGuard Home, version v0.107.7
2022/07/06 15:46:36.408149 [info] This is the first time AdGuard Home is launched
2022/07/06 15:46:36.408611 [info] Checking if AdGuard Home has necessary permissions
2022/07/06 15:46:36.409062 [fatal] This is the first launch of AdGuard Home. You must run it as Administrator.
```

### Expected Behavior
Under docker, when running AdGuardHome as a non-root user with the NET_BIND_SERVICE capability granted, AdGuardHome starts normally.

### How to reproduce
Edit AdGuardHome's Dockerfile (See this [draft PR on yscialom/AdGuardHome](https://github.com/yscialom/AdGuardHome/pull/1/files)), build and run with `docker run -e PUID=$(id -un) ...`.

Preliminary Analysis
---

### Non-root user can bind processes to ports <1024

This can be checked by running the following commands inside the docker container
```shell
$ id
uid=100(adguardhome) gid=101(adguardhome) groups=101(adguardhome),101(adguardhome)

$ getcap ../AdGuardHome
../AdGuardHome cap_net_bind_service=eip

$ nc -vlp80
listening on [::]:80 ...
```

### Source Code

This error _"This is the first launch of AdGuard Home. You must run it as Administrator."_ can be found in [`internal/home/home.go:520`](https://github.com/AdguardTeam/AdGuardHome/blob/master/internal/home/home.go#L520):
```go
if ok, err := aghnet.CanBindPrivilegedPorts(); !ok || err != nil {
log.Fatal("This is the first launch of AdGuard Home. You must run it as Administrator.")
}
```

with `CanBindPrivilegedPorts` defined in [`internal/aghnet/net_linux.go:24`](https://github.com/AdguardTeam/AdGuardHome/blob/master/internal/aghnet/net_linux.go#L24):
```go
func canBindPrivilegedPorts() (can bool, err error) {
cnbs, err := unix.PrctlRetInt(
unix.PR_CAP_AMBIENT,
unix.PR_CAP_AMBIENT_IS_SET,
unix.CAP_NET_BIND_SERVICE,
0,
0,
)
// Don't check the error because it's always nil on Linux.
adm, _ := aghos.HaveAdminRights()

return cnbs == 1 || adm, err
}
```

with `unix.PrctlRetInt` being a binding on linux' system call [prctl(2)](https://man7.org/linux/man-pages/man2/prctl.2.html) reading:
> ```
> PR_CAP_AMBIENT_IS_SET
> The prctl() call returns 1 if the capability in
> arg3 is in the ambient set and 0 if it is not.
> ```

Additional tests are necessary to determine if `unix.PrctlRetInt` is bugged, badly called, or whatever. But one can be convinced that, in this scenario, AdGuardHome should be able to open port 53 and should not exist with an error.

Why this issue matters
---

I discovered this issue while preparing a pull request to allow the docker flavour of AdGuardHome to run as a non-root user. This is indeed beneficial for two reasons:
- Running as root, even inside a container, is against security principles and can lead to privilege escalations.
- Running as root makes AdGuardHome create work files owned by root, which is detrimental to an effective administration.

Additional Information
---
### Result of `./AdGuardHome -v --version`
```
AdGuard Home
Version: v0.107.7
Channel: release
Go version: go1.17.9
Commit time: 2022-06-06 15:10:40 +0000 UTC
GOOS: linux
GOARCH: amd64
Race: false
Dependencies:
github.com/AdguardTeam/dnsproxy@v0.43.1 (sum: h1:E777KfQAi+VurOoWEdGQ5iqjSOOAzzbTfLOEzj8heCs=)
github.com/AdguardTeam/golibs@v0.10.8 (sum: h1:diU9gP9qG1qeLbAkzIwfUerpHSqzR6zaBgzvRMR/m6Q=)
github.com/AdguardTeam/urlfilter@v0.16.0 (sum: h1:IO29m+ZyQuuOnPLTzHuXj35V1DZOp1Dcryl576P2syg=)
github.com/NYTimes/gziphandler@v1.1.1 (sum: h1:ZUDjpQae29j0ryrS0u/B8HZfJBtBQHjqw2rQ2cqUQ3I=)
github.com/aead/chacha20@v0.0.0-20180709150244-8b13a72661da (sum: h1:KjTM2ks9d14ZYCvmHS9iAKVt9AyzRSqNU1qabPih5BY=)
github.com/aead/poly1305@v0.0.0-20180717145839-3fee0db0b635 (sum: h1:52m0LGchQBBVqJRyYYufQuIbVqRawmubW3OFGqK1ekw=)
github.com/ameshkov/dnscrypt/v2@v2.2.3 (sum: h1:X9UP5AHtwp46Ji+sGFfF/1Is6OPI/SjxLqhKpx0P5UI=)
github.com/ameshkov/dnsstamps@v1.0.3 (sum: h1:Srzik+J9mivH1alRACTbys2xOxs0lRH9qnTA7Y1OYVo=)
github.com/beefsack/go-rate@v0.0.0-20220214233405-116f4ca011a0 (sum: h1:0b2vaepXIfMsG++IsjHiI2p4bxALD1Y2nQKGMR5zDQM=)
github.com/cheekybits/genny@v1.0.0 (sum: h1:uGGa4nei+j20rOSeDeP5Of12XVm7TGUd4dJA9RDitfE=)
github.com/digineo/go-ipset/v2@v2.2.1 (sum: h1:k6skY+0fMqeUjjeWO/m5OuWPSZUAn7AucHMnQ1MX77g=)
github.com/fsnotify/fsnotify@v1.5.4 (sum: h1:jRbGcIw6P2Meqdwuo0H1p6JVLbL5DHKAKlYndzMwVZI=)
github.com/go-ping/ping@v0.0.0-20211130115550-779d1e919534 (sum: h1:dhy9OQKGBh4zVXbjwbxxHjRxMJtLXj3zfgpBYQaR4Q4=)
github.com/google/go-cmp@v0.5.7 (sum: h1:81/ik6ipDQS2aGcBfIN5dHDB36BwrStyeAQquSYCV4o=)
github.com/google/gopacket@v1.1.19 (sum: h1:ves8RnFZPGiFnTS0uPQStjwru6uO6h+nlr9j6fL7kF8=)
github.com/google/renameio@v1.0.1 (sum: h1:Lh/jXZmvZxb0BBeSY5VKEfidcbcbenKjZFzM/q0fSeU=)
github.com/google/uuid@v1.3.0 (sum: h1:t6JiXgmwXMjEs8VusXIJk2BXHsn+wx8BZdTaoZ5fu7I=)
github.com/insomniacslk/dhcp@v0.0.0-20220405050111-12fbdcb11b41 (sum: h1:Yg3n3AI7GoHnWt7dyjsLPU+TEuZfPAg0OdiA3MJUV6I=)
github.com/josharian/native@v1.0.0 (sum: h1:Ts/E8zCSEsG17dUqv7joXJFybuMLjQfWE04tsBODTxk=)
github.com/kardianos/service@v1.2.1 (sum: h1:AYndMsehS+ywIS6RB9KOlcXzteWUzxgMgBymJD7+BYk=)
github.com/lucas-clemente/quic-go@v0.27.1 (sum: h1:sOw+4kFSVrdWOYmUjufQ9GBVPqZ+tu+jMtXxXNmRJyk=)
github.com/marten-seemann/qtls-go1-17@v0.1.1 (sum: h1:DQjHPq+aOzUeh9/lixAGunn6rIOQyWChPSI4+hgW7jc=)
github.com/mdlayher/ethernet@v0.0.0-20220221185849-529eae5b6118 (sum: h1:2oDp6OOhLxQ9JBoUuysVz9UZ9uI6oLUbvAZu0x8o+vE=)
github.com/mdlayher/netlink@v1.6.0 (sum: h1:rOHX5yl7qnlpiVkFWoqccueppMtXzeziFjWAjLg6sz0=)
github.com/mdlayher/raw@v0.0.0-20211126142749-4eae47f3d54b (sum: h1:MHcTarUMC4sFA7eiyR8IEJ6j2PgmgXR+B9X2IIMjh7A=)
github.com/mdlayher/socket@v0.2.3 (sum: h1:XZA2X2TjdOwNoNPVPclRCURoX/hokBY8nkTmRZFEheM=)
github.com/miekg/dns@v1.1.49 (sum: h1:qe0mQU3Z/XpFeE+AEBo2rqaS1IPBJ3anmqZ4XiZJVG8=)
github.com/patrickmn/go-cache@v2.1.0+incompatible (sum: h1:HRMgzkcYKYpi3C8ajMPV8OFXaaRUnok+kx1WdO15EQc=)
github.com/pkg/errors@v0.9.1 (sum: h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=)
github.com/ti-mo/netfilter@v0.4.0 (sum: h1:rTN1nBYULDmMfDeBHZpKuNKX/bWEXQUhe02a/10orzg=)
github.com/u-root/uio@v0.0.0-20220204230159-dac05f7d2cb4 (sum: h1:hl6sK6aFgTLISijk6xIzeqnPzQcsLqqvL6vEfTPinME=)
go.etcd.io/bbolt@v1.3.6 (sum: h1:/ecaJf0sk1l4l6V4awd65v2C3ILy7MSj+s/x1ADCIMU=)
golang.org/x/crypto@v0.0.0-20220411220226-7b82a4e95df4 (sum: h1:kUhD7nTDoI3fVd9G4ORWrbV5NY0liEs/Jg2pv5f+bBA=)
golang.org/x/net@v0.0.0-20220425223048-2871e0cb64e4 (sum: h1:HVyaeDAYux4pnY+D/SiwmLOR36ewZ4iGQIIrtnuCjFA=)
golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c (sum: h1:5KslGYwFpkhGh+Q16bwMP3cOontH8FOep7tGV86Y7SQ=)
golang.org/x/sys@v0.0.0-20220422013727-9388b58f7150 (sum: h1:xHms4gcpe1YE7A3yIllJXP16CMAGuqwO2lX1mTyyRRc=)
golang.org/x/text@v0.3.7 (sum: h1:olpwvP2KacW1ZWvsR7uQhoyTYvKAupfQrRGBFM352Gk=)
gopkg.in/natefinch/lumberjack.v2@v2.0.0 (sum: h1:1Lc07Kr7qY4U2YPouBjpCLxpiyxIVoxqXgkXLknAOE8=)
gopkg.in/yaml.v2@v2.4.0 (sum: h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=)
howett.net/plist@v1.0.0 (sum: h1:7CrbWYbPPO/PyNy38b2EB/+gYbjCe2DXBxgtOOZbSQM=)
```