nixos/networking: Add the FQDN and hostname to /etc/hosts

This fixes the output of "hostname --fqdn" (previously the domain name
was not appended). Additionally it's now possible to use the FQDN.

This works by unconditionally adding two entries to /etc/hosts:
127.0.0.1 localhost
::1 localhost

These are the first two entries and therefore gethostbyaddr() will
always resolve "127.0.0.1" and "::1" back to "localhost" [0].
This works because nscd (or rather the nss-files module) returns the
first matching row from /etc/hosts (and ignores the rest).

The FQDN and hostname entries are appended later to /etc/hosts, e.g.:
127.0.0.2 nixos-unstable.test.tld nixos-unstable
::1 nixos-unstable.test.tld nixos-unstable
Note: We use 127.0.0.2 here to follow nss-myhostname (systemd) as close
as possible. This has the advantage that 127.0.0.2 can be resolved back
to the FQDN but also the drawback that applications that only listen to
127.0.0.1 (and not additionally ::1) cannot be reached via the FQDN.
If you would like this to work you can use the following configuration:
```nix
networking.hosts."127.0.0.1" = [
  "${config.networking.hostName}.${config.networking.domain}"
  config.networking.hostName
];
```

Therefore gethostbyname() resolves "nixos-unstable" to the FQDN
(canonical name): "nixos-unstable.test.tld".

Advantages over the previous behaviour:
- The FQDN will now also be resolved correctly (the entry was missing).
- E.g. the command "hostname --fqdn" will now work as expected.
Drawbacks:
- Overrides entries form the DNS (an issue if e.g. $FQDN should resolve
  to the public IP address instead of 127.0.0.1)
  - Note: This was already partly an issue as there's an entry for
    $HOSTNAME (without the domain part) that resolves to
    127.0.1.1 (!= 127.0.0.1).
- Unknown (could potentially cause other unexpected issues, but special
  care was taken).

[0]: Some applications do apparently depend on this behaviour (see
c5789246

) and this is typically the expected behaviour.

Co-authored-by: Florian Klink <flokli@flokli.de>