ShmCreateSegment asks for a file descriptor for a memory mapped file
created by the X server. This patch uses O_TMPFILE where available,
and also uses the SHMDIR directory to store the files, both for the
O_TMPFILE and mkstemp cases.
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>