nixos/gdm: follow upstream's service config

nixos/modules/services/x11/display-managers/gdm.nix

@@ -166,9 +166,15 @@ in
       "systemd-machined.service"
       "systemd-user-sessions.service"
       "getty@tty1.service"
+      "plymouth-quit.service"
+      "plymouth-start.service"
     ];
     systemd.services.display-manager.conflicts = [
       "getty@tty1.service"
+      "plymouth-quit.service"
+    ];
+    systemd.services.display-manager.onFailure = [
+      "plymouth-quit.service"
     ];
 
     systemd.services.display-manager.serviceConfig = {
@@ -178,6 +184,9 @@ in
       BusName = "org.gnome.DisplayManager";
       StandardOutput = "syslog";
       StandardError = "inherit";
+      ExecReload = "${pkgs.coreutils}/bin/kill -SIGHUP $MAINPID";
+      KeyringMode = "shared";
+      EnvironmentFile = "-/etc/locale.conf";
     };
 
     systemd.services.display-manager.path = [ pkgs.gnome3.gnome-session ];