PDP-11 support

Change KMC to KDP in autoconfig table.

Add KMC_UNITS to pdp11_defs so they can be enabled.  2 for now.
pdp11 NPR to IOPAGE

Add (as yet untested) support for PDP11 (and thus VAX) Unibus devices to
read and write registers in the IO page.

If SimH had an ODT ROM, I'd test it...