kernel - Redo struct vmspace allocator and ref-count handling.

* Get rid of the sysref-based allocator and ref-count handler and
  replace with objcache.  Replace all sysref API calls in other kernel
  modules with vmspace_*() API calls (adding new API calls as needed).

* Roll-our-own hopefully safer ref-count handling.  We get rid of exitingcnt
  and instead just leave holdcnt bumped during the exit/reap sequence.  We
  add vm_refcnt and redo vm_holdcnt.

  Now a formal reference (vm_refcnt) is ALSO covered by a holdcnt.  Stage-1
  termination occurs when vm_refcnt transitions from 1->0.  Stage-2 termination
  occurs when vm_holdcnt transitions from 1->0.

* Should fix rare reported panic under heavy load.