main: Force Xwayland startup if not on systemd

In non-systemd managed session we are unable to start services on
demand. Instead, gnome-session will start everything at login time,
including any X11 related service (i.e. gsd-xsettings).

However, in order to start gsd-xsettings, Xwayland needs to be started
already. Otherwise it will connect to GNOME_SETUP_DISPLAY and login will
hang at that point.

Fix this by detecting whether mutter is running in a systemd unit. If it
is, we assume that we are systemd managed and the machinery to start the
services works fine. If not, we assume that the session management may
unconditionally try to start X11 related services and Xwayland must be
started in order to not block this.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1771>

src/core/main.c

@@ -1014,11 +1014,18 @@ meta_get_x11_display_policy (void)
 #ifdef HAVE_WAYLAND
   if (meta_is_wayland_compositor ())
     {
+#ifdef HAVE_XWAYLAND_INITFD
+      g_autofree char *unit = NULL;
+#endif
+
       if (opt_no_x11)
         return META_DISPLAY_POLICY_DISABLED;
 
 #ifdef HAVE_XWAYLAND_INITFD
-      return META_DISPLAY_POLICY_ON_DEMAND;
+      if (sd_pid_get_user_unit (0, &unit) < 0)
+        return META_DISPLAY_POLICY_MANDATORY;
+      else
+        return META_DISPLAY_POLICY_ON_DEMAND;
 #endif
     }
 #endif