Fix bugs in dealing with low-memory situations when the system has run out

Fix bugs in dealing with low-memory situations when the system has run out
of swap or has no swap.

* Fix an error where the system started killing processes before it needed
  to.

* Continue propagating pages from the active queue to the inactive queue
  when the system has run out of swap or has no swap, even though the
  inactive queue has become bloated.  This occurs because the inactive
  queue may be unable to drain due to an excess of dirty pages which
  cannot be swapped out.

* Use the active queue to detect excessive stress which combined with
  an out-of-swap or no-swap situation means the system has run out of
  memory.  THEN start killing processes.

* This also allows the system to recycle nearly all the clean pages
  available when it has no swap space left, to try to keep things going,
  leaving only dirty pages in the VM page queues.