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.

This -initfd can be seen as a generalization of -wm, a Wayland
compositor may use -initfd to launch its WM and any other clients
that should start up, or it may use -wm as a dedicated connection for
the WM and optionally use -initfd for the misc. startup clients.

If either of -wm or -initfd is passed, Xwayland will expect a selection
notification on WM_S0 before incorporating the FDs in -listen to the
poll list.

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

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
3 jobs for xwayland-init-socket in 5 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Docker Image
passed #449820
debian-testing

00:00:20

 
  Build And Test
passed #449821
autotools-build-and-test

00:05:30

passed #449822
meson-build-and-test

00:01:56