xwayland: Raise the FD limit to the max

Xwayland may open a fair amount of file descriptors for passing Wayland
buffers, even more so when using the `wl_shm` either for the pointer
cursors or for when GLAMOR is not usable.

As a result, Xwayland may hit the (soft) limit of file descriptors
leading to a Wayland protocol error and the termination of Xwayland.

To mitigate that risk, raise the limit to the maximum (hard) limit of
file descriptors (unless of course the limit was set explicitly from the
command line with "-lf").

Note that for completeness, the Wayland compositor may have to do the
same, otherwise the limit might get reached on the compositor side as

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Simon Ser <contact@emersion.fr>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Closes: #1283
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