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