xwayland: Manually create sockets

This provides kwin greater control over how X11 sockets are created for
Xwayland. For example, it can be used to ensure that the DISPLAY remains
the same across Xwayland server restarts or launching Xwayland on
demand.

Even though -listen <fd> option is deprecated, we still pass it because
older versions of Xwayland may not have the -listenfd option.