- Add a statically allocated memguard arena since it is needed very early
   on.
 - Pass the appropriate flags to vmem_xalloc() when allocating space for
   the arena from kmem_arena.

Sponsored by:	EMC / Isilon Storage Division
 - Disable quantum caches on the kmem_arena.  This can make fragmentation
   worse on small KVA systems.  I had intended to only enable it for
   debugging.

Sponsored by:	EMC / Isilon Storage Division