epoch(9): fix priority handling, make callback lists pcpu, and other fixes

epoch(9): fix priority handling, make callback lists pcpu, and other fixes

- Lend priority to preempted threads in epoch_wait to handle the case
  in which we've had priority lent to us. Previously we borrowed the
  priority of the lowest priority preempted thread. (pointed out by mjg@)

- Don't attempt allocate memory per-domain on powerpc, we don't currently
  handle empty sockets (as is the case on jhibbits Talos' board).

- Handle deferred callbacks as pcpu lists and poll the lists periodically.
  Currently the interval is 1/hz.

- Drop the thread lock when adaptive spinning. Holding the lock starves
  other threads and can even lead to lockups.

- Keep a generation count pcpu so that we don't keep spining if a thread
  has left and re-entered an epoch section.

- Actually removed the callback from the callback list so that we don't
  double free. Sigh ...

Approved by:	sbruno@