switch-to-configuration: Don't restart instances of user@.service

Restarting user@ instances is bad because it causes all user services
(such as ssh-agent.service) to be restarted. Maybe one day we can have
switch-to-configuration restart user units in a fine-grained way, but
for now we should just ignore user systemd instances.

Backport: 14.04

nixos/modules/system/boot/systemd.nix

@@ -812,5 +812,7 @@ in
         ${concatStringsSep "\n" cfg.tmpfiles.rules}
       '';
 
+    systemd.services."user@".restartIfChanged = false;
+
   };
 }