network-link-*.service: Set stopIfChanged = false

This reduces the time window during which IP addresses are gone during
switch-to-configuration. A complication is that with stopIfChanged =
true, preStop would try to delete the *new* IP addresses rather than
the old one (since the preStop script now runs after the switch to the
new configuration). So we now record the actually configured addresses
in /run/nixos/network/addresses/<interface>. This is more robust in
any case.

Issue https://github.com/NixOS/nixops/issues/640.

nixos/modules/security/polkit.nix

@@ -64,7 +64,7 @@ in
     systemd.packages = [ pkgs.polkit.out ];
 
     systemd.services.polkit.restartTriggers = [ config.system.path ];
-    systemd.services.polkit.unitConfig.X-StopIfChanged = false;
+    systemd.services.polkit.stopIfChanged = false;
 
     # The polkit daemon reads action/rule files
     environment.pathsToLink = [ "/share/polkit-1" ];