This commit adds a "PMMI" device to the AltairZ80 VM that emulates
the PMMI Communications MM-103 MODEM & Communications Adapter.
This device was added at the request of a Facebook user that is
attempting to preserve and document various relevant communications
programs for CP/M and allow users to experience communicating and
sharing information with other remote computers of the same time
period, all on modern hardware.
This device attempts to emulate the MC6860L digital modem chip in
such a way that communications software written for the PMMI MM-103 can
communicate over a serial port or socket without a phone line or
the ability to MOdulate/DEModulate data over a phone line.
