AltairZ80: Changes to JAIR devices

[deltecent]

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 (thanks @markpizz)
• Adds a 128-byte input ring buffer
• Corrects a null pointer exception
• Adds overrun status bit implementation
• Replaces a tab with spaces

The simulator will now boot Mike Douglas' Serial CP/M, modified for the JAIR, over a serial port connected to the FDC+ serial server at 57.6K.

ALTAIR/IMSAI 8080 CPU BOARD BOOT LOADER - Josh Bensadon v2.6 May 30, 2024

INIT_FAT 
Init SD Type#1 ACMD41+MBR S Type06 PBR S VOL=JAIRCPM     SYS=FAT16   

<D> -SD Card Directory
<R> -RAM Test
<S> -Serial Boot
<V> -View Load
<1> -1.HEX
<2> -2.HEX
<3> -3.HEX
<4> -BIOS61K.HEX
<5> -BIOS.HEX
<6> -4.HEX
<7> -5.HEX
<8> -6.HEX
<9> -7.HEX
> s
JAIR FDC+ Serial Server Boot Loader (COM2 at 57.6K)
Requesting TRACK 0 from Serial Server...
Track received.

Execute at:0000
..

56K CP/M 2.2 v1.0 JAIR
For SERIAL Altair 8" Floppy

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