Merge the vm_page hold and wire mechanisms.

Merge the vm_page hold and wire mechanisms.

The hold_count and wire_count fields of struct vm_page are separate
reference counters with similar semantics.  The remaining essential
differences are that holds are not counted as a reference with respect
to LRU, and holds have an implicit free-on-last unhold semantic whereas
vm_page_unwire() callers must explicitly determine whether to free the
page once the last reference to the page is released.

This change removes the KPIs which directly manipulate hold_count.
Functions such as vm_fault_quick_hold_pages() now return wired pages
instead.  Since r328977 the overhead of maintaining LRU for wired pages
is lower, and in many cases vm_fault_quick_hold_pages() callers would
swap holds for wirings on the returned pages anyway, so with this change
we remove a number of page lock acquisitions.

No functional change is intended.  __FreeBSD_version is bumped.

Reviewed by:	alc, kib
Discussed with:	jeff
Discussed with:	jhb, np (cxgbe)
Tested by:	pho (previous version)
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D19247