networkd: Fix evaluation of systemd.network units.

During the refactor of the networkd stuff in f8dbe5f3

, a lot of the
options are now needed by systemd.nix as well as networkd.nix but
weren't moved by that commit as well.

For now, this fixes all networkd VM tests except for the macvlan one and
thus it should fix #7505 for at least DHCP-based configuration.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>