Implement the populate() pager method for phys pager.

It allows to provide configurable agressive prefaulting and useful
hints to page daemon about memory allocations, on faults for pages
managed by phys pager. In fact, this implementation is superior to
the MAP_SHARED_PHYS hack from my Postgresql paper, while giving
similar benefits of reducing the page faults numbers on SysV shared
memory mappings.

Reviewed by: alc
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks