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.