Don't include networkd units unless enabled

Otherwise, the enabled -> disabled transition won't be handled
correctly (switch-to-configuration currently assumes that if a unit is
running and exists, it should be restarted).

nixos/modules/system/boot/networkd.nix

···
 
   config = mkIf config.systemd.network.enable {
 
+    systemd.additionalUpstreamSystemUnits =
+      [ "systemd-networkd.service" "systemd-networkd-wait-online.service" ];
+
     systemd.network.units =
       mapAttrs' (n: v: nameValuePair "${n}.link" (linkToUnit n v)) cfg.links
       // mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs

nixos/modules/system/boot/resolved.nix

···
 
   config = mkIf config.services.resolved.enable {
 
+    systemd.additionalUpstreamSystemUnits = [ "systemd-resolved.service" ];
+
     systemd.services.systemd-resolved = {
       wantedBy = [ "multi-user.target" ];
       restartTriggers = [ config.environment.etc."systemd/resolved.conf".source ];

nixos/modules/system/boot/systemd.nix

···
       "systemd-modules-load.service"
       "kmod-static-nodes.service"
 
-      # Networking
-      "systemd-networkd.service"
-      "systemd-networkd-wait-online.service"
-      "systemd-resolved.service"
-      "systemd-timesyncd.service"
-
       # Filesystems.
       "systemd-fsck@.service"
       "systemd-fsck-root.service"

nixos/modules/system/boot/timesyncd.nix

···
 
   config = mkIf config.services.timesyncd.enable {
 
+    systemd.additionalUpstreamSystemUnits = [ "systemd-timesyncd.service" ];
+
     systemd.services.systemd-timesyncd = {
       wantedBy = [ "sysinit.target" ];
       restartTriggers = [ config.environment.etc."systemd/timesyncd.conf".source ];