[Backport to 3-38] 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 writeable and has the sticky bit.