[Simh] IMP interfaces

[Phil Budne]

There are many reasons for supporting "1822" (the host-to-imp
protocol) in SimH in a generic way:

To speak to a simulated pre-TCP-era (aka NCP) IMP
   (or a simulated TCP-era IMP if that code becomes available)

To speak to the TCP Internet on a simulated system that never had an
Ethernet interface.

BOTH are meaningful for ITS, but I think it's the latter that Lars is
pursuing at the moment.

It would be lovely if there was a common API for 1822 communications
(whether to an external simulation running IMP code, or an internal
simulated IMP that allows the host to talk to the Internet)

p