- Change the wakeup logic associated with having multiple sleepers

- Change the wakeup logic associated with having multiple sleepers
  on multiple different audit pipes.  The old method used cv_signal()
  which would result in only one thread being woken up after we
  appended a record to it's queue.  This resulted in un-timely wake-ups
  when processing audit records real-time.

- Assign PSOCK priority to threads that have been sleeping on a read(2).
  This is the same priority threads are woken up with when they select(2)
  or poll(2).  This yields fairness between various forms of sleep on
  the audit pipes.

Obtained from:	TrustedBSD Project
Discussed with:	rwatson
MFC after:	1 week