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.