nixos: Fix ordering of firewall.service

Follow-up to the following commits:

  abdc5961c3: Fix starting the firewall
  e090701e2d: 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

nixos/modules/services/networking/firewall.nix

@@ -490,7 +490,7 @@ in
 
     systemd.services.firewall = {
       description = "Firewall";
-      wantedBy = [ "multi-user.target" "sysinit.target" ];
+      wantedBy = [ "sysinit.target" ];
       wants = [ "network-pre.target" ];
       before = [ "network-pre.target" ];
       after = [ "systemd-modules-load.service" ];
@@ -501,6 +501,7 @@ in
       # containers don't have CAP_SYS_MODULE. So the host system had
       # better have all necessary modules already loaded.
       unitConfig.ConditionCapability = "CAP_NET_ADMIN";
+      unitConfig.DefaultDependencies = false;
 
       reloadIfChanged = true;