o push much of the i386 and amd64 MD interrupt handling code
  (intr_machdep.c::intr_execute_handlers()) into MI code
  (kern_intr.c::ithread_loop())
o move filter handling to kern_intr.c::intr_filter_loop()
o factor out the code necessary to mask and ack an interrupt event
  (intr_machdep.c::intr_eoi_src() and intr_machdep.c::intr_disab_eoi_src()),
  and make them part of 'struct intr_event', passing them as arguments to
  kern_intr.c::intr_event_create().
o spawn a private ithread per handler (struct intr_handler::ih_thread)
  with filter and ithread functions.

Approved by: re (implicit?)