handlers. (Only slot_ignore() and slot_noint() need this, and we already
have a place to put this information.) Adjust add_nubus_intr() so that if
the client_data arg is specified as NULL, pass the slot number as
client_data to the interrupt handler.
Also change the device probing scheme to use something a bit more rational.
A current side-effect is that nubus cards are double-mapped. I expect
to fix that shortly.
Also change splclock() to block everything but serial hardware interrupts.