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 to the same, otherwise the limit might get reached on the compositor side as well.