Add a shm_open2 syscall to support upcoming memfd_create

shm_open2 allows a little more flexibility than the original shm_open.
shm_open2 doesn't enforce CLOEXEC on its callers, and it has a separate
shmflag argument that can be expanded later. Currently the only shmflag is
to allow file sealing on the returned fd.

shm_open and memfd_create will both be implemented in libc to use this new
syscall.

__FreeBSD_version is bumped to indicate the presence.

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