xwayland: Check permissions on /tmp/.X11-unix

For Xwayland, mutter creates the sockets in the standard /tmp/.X11-unix
directory.

Yet, if that directory already exists, it may have been created by
another user with full control over the created socket.

To avoid that issue, if the directory /tmp/.X11-unix already exists,
check that the permissions are as we expect, i.e. the directory belongs
to either root or the user herself, is writable and has the sticky bit.

Thanks to fabian@ritter-vogt.de for reporting that issue.

https://gitlab.gnome.org/GNOME/mutter/-/issues/1708
(cherry picked from commit 3a08fcf2accb4b42fe85f98ddf83223200db928e)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1788>