Restore pmap_copy() for 4/4 i386 pmap.

Create yet another temporal pte mapping routine pmap_pte_quick3(),
which is the copy of the pmap_pte_quick() and relies on the
pvh_global_lock to protect the frame. It accounts into the same
counters as pmap_pte_quick(). It is needed since pmap_copy() uses
pmap_pte_quick() already, and since a user pmap is no longer current
pmap.

pmap_copy() still provides the advantage for real-world workloads
involving lot of forks where processes do not exec immediately.

Benchmarked by: bde
Sponsored by: The FreeBSD Foundation