rejecting INTR_FAST interrupts. Since they can't be shared anyway,
this just short circuits a failure case that should work but is panic
fodder now.
This bug is that if the interrut condiation is active when you activate
the interrupt, then the interrupt routine will be called. jhb had
a patch that may or may not work to fix it, but I've lost it.
This may be due to the sio probe doing something odd too.
9c03640f81