o Add accessor functions to add and remove pages from a specific

o Add accessor functions to add and remove pages from a specific
  freelist.
o Split the pool of free pages queues really by domain and not rely on
  definition of VM_RAW_NFREELIST.
o For MAXMEMDOM > 1, wrap the RR allocation logic into a specific
  function that is called when calculating the allocation domain.
  The RR counter is kept, currently, per-thread.
  In the future it is expected that such function evolves in a real
  policy decision referee, based on specific informations retrieved by
  per-thread and per-vm_object attributes.
o Add the concept of "probed domains" under the form of vm_ndomains.
  It is responsibility for every architecture willing to support multiple
  memory domains to correctly probe vm_ndomains along with mem_affinity
  segments attributes.  Those two values are supposed to remain always
  consistent.
  Please also note that vm_ndomains and td_dom_rr_idx are both int
  because segments already store domains as int.  Ideally u_int would
  have much more sense. Probabilly this should be cleaned up in the
  future.
o Apply RR domain selection also to vm_phys_zero_pages_idle().

Sponsored by:	EMC / Isilon storage division
Partly obtained from:	jeff
Reviewed by:	alc
Tested by:	jeff