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.
Status Job ID Name Coverage
  Docker Image
passed #434127
debian-testing

00:00:32

 
  Build And Test
passed #434128
autotools-build-and-test

00:05:26

passed #434129
meson-build-and-test

00:02:05