wayland: make Xwayland listen on abstract namespace socket again

This is intended to fix #1454 (closed). It reverts the removal of the abstract namespace X11 socket, and adjusts the on demand launch code to watch both the abstract and regular X11 sockets.

This was enough to get snap applications running in a Wayland gnome-shell session. I'm not sure of the best way to test that the on demand launching behaviour is correct for both abstract and regular sockets though.