rather than assigning to the whole field, set or clear individual flags,
which implies that the B_BUSY and B_INVAL flags will remain set.
this allows us to make the assertion in brelse() that B_BUSY is set,
which is the purpose of all this.
Move ivec_dsp from trap.h to scb.h
Add interrupt counting to asc, le, ze, ncr, dz interrupts.
add common support for counting interrupts to cnm_idisp so
it can be done very efficiently.
change these from bp->b_un.b_addr to bp->b_data, as well. This also
allows us more flexibility to experiment with other data buffer types
hung off of struct buf.
chs