xwayland: use -displayfd instead of USR1 to signal readiness

We want to wait for Xwayland to be ready before issuing it blocking
requests, but relying on USR1 is a bit unsafe:
 - we can't ascertain the signal originated from Xwayland
 - if weston is started as PID1 (e.g. in its own container), then
   Xwayland will not send SIGUSR1 and X11 connections will be stuck
   forever: xorg/xserver#1312



Creating a pipe and using -displayfd, even if we don't care about the
display value itself, is safe and works for all cases

Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>