Use memfd for shm buffers

Wanted to do this a while back but completely forgot.

This makes shm handling simpler as the compositor doesn't have to worry about the SIGBUS handler.

See sigbus_is_impossible in https://gitlab.freedesktop.org/wayland/wayland/-/blob/main/src/wayland-shm.c#L325

This would have avoided phoc#290 (closed) as well (but it's good to have that sorted on the compositor side so clients can't trigger it). https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/474 is a good reference.

I added a warning for linux without memfd_create.

I got reminded of this by https://lwn.net/Articles/918106/