kernel - Refactor kqueue interlocks

kernel - Refactor kqueue interlocks

* Make KN_PROCESSING a soft lock flag.  When set nobody else can mess
  with a particular knote (other than setting certain flags) even if
  the originator blocks.

* Interlock major processing with KN_PROCESSING.  Registration, event
  scan, knote(), deletion, and filter ops.

* Block & restart when conflicts occur.  For the knote() hot-path we only
  block and restart if the 'hint' is non-zero, otherwise we just flag with
  KN_REPROCESS to indicate that reprocessing is required.

* This should fix kqueue races related to blocking operations confusing
  the list scan.

* Document the shit out of everything.

Reported-by: Francois Tigeot <ftigeot@wolfpond.org>