networkd: allow to supply own unit files

Networkd units can contain secrets. In future also wireguard vpn will be supported by
networkd. To avoid leakage of private keys, those could be then also put outside
of the /nix/store

Having a writeable /etc/systemd/network also allows to quick fix network issues,
when upgrading `nixos-rebuild switch` would require network on its own (due
updates).