kernel - kqueue - Bug fixing pass

kernel - kqueue - Bug fixing pass

* We cannot terminate the loop when events are received if all the
  events are spurious (none were collected by the copyout).

* Move the WOULDBLOCK -> 0 handling out of kqueue_scan() and into
  kern_kevent().

* Preset (*errorp) to 0 so it is set definitively for all cases, including
  degenerate cases.

* Allow an infinite number of events when doselect() calls kern_kevent()
  to avoid premature termination on successive calls which could occur
  due to spurious events.

* Fix a bug in dopoll() where the bytes variable was calculated prior to
  the truncation of nfds.

* Fix copyin/copyout arguments.

* Add missing rel_mplock() in pipe_kqfilter()