nixos/xdg/autostart: actually disable generation of autostart units

This did not actually do anything when set to false.

Fixes https://github.com/NixOS/nixpkgs/issues/380166

I split this into two options that are independent of each other:

1. Whether to link the `systemPackages`' autostarts
2. Whether to enable their interpretation

(You can have interpretation of user-provided autostarts and the user could
interpret the system-provided autostarts.)

nixos/modules/config/xdg/autostart.nix

@@ -4,21 +4,33 @@
     teams = [ lib.teams.freedesktop ];
   };
 
-  options = {
-    xdg.autostart.enable = lib.mkOption {
-      type = lib.types.bool;
-      default = true;
-      description = ''
-        Whether to install files to support the
-        [XDG Autostart specification](https://specifications.freedesktop.org/autostart-spec/latest).
-      '';
-    };
+  options.xdg.autostart = {
+    enable =
+      lib.mkEnableOption "auto-starting of desktop applications according to the [XDG Autostart specification](https://specifications.freedesktop.org/autostart-spec/latest)."
+      // lib.mkOption {
+        default = true;
+      };
+    install =
+      lib.mkEnableOption ''
+        install desktop files following the [XDG Autostart specification](https://specifications.freedesktop.org/autostart-spec/latest) into `/etc/xdg/autostart/`.
+
+        These are handled by your desktop environment or [`systemd-xdg-autostart-generator`](https://www.freedesktop.org/software/systemd/man/latest/systemd-xdg-autostart-generator.html).
+      ''
+      // lib.mkOption {
+        default = true;
+      };
   };
 
-  config = lib.mkIf config.xdg.autostart.enable {
-    environment.pathsToLink = [
+  config = {
+    # FIXME this does not actually work because "/etc/xdg" is linked
+    # unconditionally in `nixos/modules/config/system-path.nix`
+    environment.pathsToLink = lib.mkIf config.xdg.autostart.install [
       "/etc/xdg/autostart"
     ];
-  };
 
+    # On by default
+    systemd.user.generators.systemd-xdg-autostart-generator = lib.mkIf (!config.xdg.autostart.enable) (
+      lib.mkDefault "/dev/null"
+    );
+  };
 }