Bring back r1.78 and r1.79, now that ajactouto@'s regression has

Bring back r1.78 and r1.79, now that ajactouto@'s regression has
been found: it was a hardware failure.

When a bus is explored, do not probe the ports which status hasn't
changed.  This saves a lot of I/O when attaching/detaching devices
and might help with some timing related problems.