Fix starting the firewall

Probably as a result of 992c514a, it
was not being started anymore.

My understanding of systemd.special(7) (section "Special passive
system units") is that the firewall should want network-pre.target,
rather than the other way around (not very intuitive...). This in
itself does not cause the firewall to be wanted, which is why the
wanted-by relationship with multi-user.target is necessary.

http://hydra.nixos.org/build/39965589