kernel - Fix a major (pageable) memory leak

* Under certain relatively easy to reproduce conditions an extra ref_count
  can be added to a VM object during a fork(), preventing the object from
  ever being destroyed.  It's pages may even be paged out, but the system
  will eventually run out of swap space too.

* The actual fix is to assign 'map_object = object' in vm_map_insert()
  (see the diff).  The rest of this commit is conditionalized debugging
  code and code documentation.

* Because this change implements a relatively esoteric feature in the VM
  system by allowing an anonymous VM object to be extended to cover an
  area even though it might have a gap (so a new VM object does not have
  to be allocated), further testing is needed before we can MFC this to
  the RELEASE branch.
swapcache - Allow cleaning to proceed if disabled

* Allow swapcache cleaning to proceed even if swapcache
  is disabled.