[Simh] Terminal mux structure(s) (was DZ vs DZV)

[Bob Supnik]

The original DZ emulator was originally written by a contributor. He 
chose a model whereby lines were not explicitly represented by units. In 
fact, even multiple controllers were not explicitly represented. This 
model was followed in some other emulators, such as the DHU/V11 and the 
NOVA QTY.

When I started writing terminal multiplexors, I was working with devices 
that were essentially multiple instances of the same simple interface, 
such as the LT15, DL11, and so on. I chose to have a unit per line, but 
still only one (SIMH-visible) controller. This allowed fine-grained 
granularity over the number of lines. I carried this model over to 
"real" terminal multiplexors on the IBM 7094, SDS systems, and so on.

Both models seem to work or can be made to work. Neither creates a 
SIMH-visible controller per real controller.

With respect to the DZ11/DZV11, I expect it would be possible to 
parameterize the "lines per controller" math and make it U/Q sensitive. 
If the CPU model is changed mid-simulation, there would be significant 
trauma to any running software, but users should realize there will 
trouble if they change a CPU on the fly.

Unless configuration limitations prevent some family of software from 
running, I don't regard the limitations as significant.

/Bob