Port the new PV entry allocator from amd64/i386. This allocator has two

Port the new PV entry allocator from amd64/i386.  This allocator has two
advantages.  First, PV entries are roughly half the size.  Second, this
allocator doesn't access the paging queues, and thus it will allow for the
removal of the page queues lock from this pmap.

Fix a rather serious bug in pmap_remove_write().  After removing write
access from the specified page's first mapping, pmap_remove_write() then
used the wrong "next" pointer.  Consequently, the page's second, third,
etc. mappings were not write protected.

Tested by:	jchandra