xwayland: Add "-listenfd" option

Using the existing command line option "-listen" for passing file
descriptors between the Wayland compositor and Xwayland is misleading,
Xwayland should add is own command line option for that specific use.

As XWayland is spawned by the Wayland compositor, we cannot just change
the option, as that would break all existing Wayland compositors using
Xwayland, so we add a new options "-listenfd" and mark the previous one
as deprecated and log a warning, but it still works for backward
compatibility.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
xorg/xserver!214
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