shared: Use memfd_create() when available
This (so-far) Linux-only API lets users create file descriptors purely in memory, without any backing file on the filesystem and the race condition which could ensue when unlink()ing it.
It also allows seals to be placed on the file, ensuring to every other process that we won’t be allowed to shrink the contents, potentially causing a SIGBUS when they try reading it.
This patch is best viewed with the -w option of git log -p.
It is an almost exact copy of Wayland commit 6908c8c85a2e33e5654f64a55cd4f847bf385cae, see wayland!4 (merged)
Signed-off-by: Emmanuel Gil Peyrot email@example.com