Add MAP_GUARD and use it for stack grow area protection.
Bump __FreeBSD_version.

MFC r320317:
Implement address space guards.

MFC r320338:
Remove stale part of the comment.

MFC r320339:
Correctly handle small MAP_STACK requests.

MFC r320344:
For now, allow mprotect(2) over the guards to succeed regardless of
the requested protection.

MFC r320430:
Treat the addr argument for mmap(2) request without MAP_FIXED flag as
a hint.

MFC r320560 (by alc):
Modify vm_map_growstack() to protect itself from the possibility of the
gap entry in the vm map being smaller than the sysctl-derived stack guard
size.