PDP11: Addition of the RS03/RS04 disk devices from Bob Supnik

Note that this is not the fully general Massbus implementation people were dreaming of. MB#1 is for RP/RM, MB#2 is for TU, and MB#3 is for RS.
I know that there were requests for four Massbus channels and multiple RP/RP subcontrollers, but if people want more disks, they can use the RQ.