xwayland: Allow passing a fd for set up clients

This fd also triggers the "wait for WM_S0" paths, so that the compositor may set up a "maintenance line" for Xwayland, for services that are essential to run before any client (eg. xrdb). Those services would use this fd, disguised as an extra display connection.

Also, correct a minor typo in the listenfd argument output, give → given.