nixos: Fix ordering of firewall.service

Follow-up to the following commits:

  abdc5961: Fix starting the firewall
  e090701e

: Order before sysinit

Solely use sysinit.target here instead of multi-user.target because we
want to make sure that the iptables rules are applied *before* any
socket units are started.

The reason I've dropped the wantedBy on multi-user.target is that
sysinit.target is already a part of the dependency chain of
multi-user.target.

To make sure that this holds true, I've added a small test case to
ensure that during switch of the configuration the firewall.service is
considered as well.

Tested using the firewall NixOS test.

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