wayland: Do not fail on stalled .X11-unix entries

If for whatever reason, there are stalled files in /tmp/.X11-unix/ the
bind() to the abstract socket will succeed but not the bind() to the
to the UNIX socket.

This causes gnome-shell/mutter to fail because it cannot start Xwayland
(while it could actually, by using a different display).

In case of failure to bind to the UNIX socket, try the next display
instead of failing, to avoid stalled entries in /tmp/.X11-unix.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/13

src/wayland/meta-xwayland.c

@@ -465,7 +465,8 @@ choose_xdisplay (MetaXWaylandManager *manager)
         {
           unlink (lock_file);
           close (manager->abstract_fd);
-          return FALSE;
+          display++;
+          continue;
         }
 
       break;