kernel - Refactor vm_page_alloc() to improve hot pages
* Since we do not pre-zero pages any more, nothing uses PG_ZERO and we no
longer have to use the head and tail of the free queue as an indication
of a possibly freed page being zerod or not.
For PQ_FREE, refactor the use case to always be LIFO, improving chances
that vm_page_alloc() will retrieve a page that is already hot in the
cache.
* Should improve the host, and in particular will improve the vkernel,
preventing it from unnecessarily cycling new pages (faulting them in
on the host) when normal operation is in steady-state, and also reducing
unnecessary initial/fresh faults.