chained podule interrupt handlers.
Fill out the ih_maskaddr and ih_maskbits fields in the interrupt handler
structure so that interrupt dispatcher knows where to look to determine
if the handler should be called.
- split softc size and match/attach out from cfdriver into
a new struct cfattach.
- new "attach" directive for files.*. May specify the name of
the cfattach structure, so that devices may be easily attached
to parents with different autoconfiguration semantics.