Revert "Revert "gdm module: only make xserver args overrideable""

This reverts commit 4e57e7f7c6.

This actually broke gnome3 and didn't fix anything, I failed bisecting.

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

@@ -99,7 +99,8 @@ in
     services.xserver.displayManager.job =
       {
         environment = {
-          GDM_X_SERVER = "${cfg.xserverBin} ${toString cfg.xserverArgs}";
+          GDM_X_SERVER_EXTRA_ARGS = toString
+            (filter (arg: arg != "-terminate") cfg.xserverArgs);
           GDM_SESSIONS_DIR = "${cfg.session.desktops}";
           # Find the mouse
           XCURSOR_PATH = "~/.icons:${config.system.path}/share/icons";