xwayland: Restore abstract socket support + do not rely on X-lock files + some other Xwayland fixes

Some X11 servers may not always create a lock file, yet mutter uses the lock file to find a possible display number and then tries to bind to the socket corresponding to that display number.

If it fails to bind, it will simply bail out. As a result, if an X11 server is already listening on that display but hadn't created a lock file, mutter won't be able to start Xwayland.

To avoid that possible issue, make mutter retry with another display for a given number of tries when binding fails even though the display was supposed to be available based on the lock file presence.