i wasn't clear enough what i meant by `interrupt handler'. i didn't mean
the device-specific function, but its caller. in pc/trap.c you see
			if(up && !clockintr)
				preempted();
and on other platforms (perhaps not enough) there are calls to preempted()
in the interrupt handler(s) that despatch the device-specific ones.
preempted calls sched, which can switch to another process before returning.