Merge MAP_GUARD.

MFC r316687 (by markj), r320314, r320317, r320338, r320339, r320344, r320430,
r320560 (by alc), r320801, r320843, r321173, r321230.

Tested by: pho
Sponsored by: The FreeBSD Foundation

MFC r315272, r315370

r315272:
Implement INHERIT_ZERO for minherit(2).

INHERIT_ZERO is an OpenBSD feature.

When a page is marked as such, it would be zeroed
upon fork().

This would be used in new arc4random(3) functions.

PR: 182610
Reviewed by: kib (earlier version)
Differential Revision: https://reviews.freebsd.org/D427

r315370:
The adj_free and max_free values of new_entry will be calculated and
assigned by subsequent vm_map_entry_link(), therefore, remove the
pointless copying.

Submitted by: alc

MFC 270823,270825,270829:
Use a unit number allocator to provide suitable st_dev and st_ino values
for POSIX shared memory descriptors. The implementation is similar to
that used for pipes.

Approved by: re (gjb for 10)

MFC r267630:
Add MAP_EXCL flag for mmap(2).

MFC 270823,270825,270829:
Use a unit number allocator to provide suitable st_dev and st_ino values
for POSIX shared memory descriptors. The implementation is similar to
that used for pipes.

Approved by: re (gjb for 10)

MFC r267630:
Add MAP_EXCL flag for mmap(2).