kernel - Count only non-spurious events in main kevent loop

kernel - Count only non-spurious events in main kevent loop

* Add a couple of debug fields to kern.nseldebug kprintf's

* The main kevent loop is effectively event driven until it has processed the
  maximum number of events it is allowed to return. Spurious events counted
  toward this total, resulting in no events being returned if the number of
  spurious events processed reached the total.