add prototyped declarations for the callbacks which add old-style
(4.4bsd/pmax conf.old) SCSI controllers, and which configure SCSI slaves
on such controllers.
rewriting the SII driver to work with the MI scsi would be a better option,
if time were available, and the MI scsi code supported SCSI controllers with
an 8k upper bound on transfers.
"struct pmax_device" to avoid conflict with <sys/device.h>.
Change the signature of interrupt-handlers to take a void *
(a pointer to the softc) and return an int (indicating spurious
interrupts or other conditions.)
