AltairZ80: Changes to JAIR devices

I was having problems with the JAIR device keeping up with received
data at 57.6K.

This PR makes the following changes to the JAIR devices:

- Refactors the service routines into STAT, TX and RX units
- Adds a 128-byte input ring buffer
- Corrects a null pointer exception
- Adds overrun status bit implementation
- Removes a tab with spaces