kernel - Fix numerous performance problems with the pageout daemon

kernel - Fix numerous performance problems with the pageout daemon

* The VM page queues were not being fully utilized, causing the pageout
  daemon to calculate incorrect average page counts for deactivation/freeing.
  This caused the pageout daemon to dig into the active queue even when it
  did not need to.

* The pageout daemon was incorrectly calculating the maxscan value for each
  queue.  It was using the aggregate count (across all 256 queues) instead of
  the per-queue count, resulting in long stalls when memory is low.

* Clean up the PQ_L2* knobs, constants, and other cruft, reducing them to
  the essentials for our goals.

Reported-by: vsrinivas, thesjg, luxh, etc