Add linux-compatible memfd_create

memfd_create is effectively a SHM_ANON shm_open(2) mapping with optional
CLOEXEC and file sealing support. This is used by some mesa parts, some
linux libs, and qemu can also take advantage of it and uses the sealing to
prevent resizing the region.

This reimplements shm_open in terms of shm_open2(2) at the same time.

shm_open(2) will be moved to COMPAT12 shortly.

Reviewed by: markj, kib
Differential Revision: https://reviews.freebsd.org/D21393