Implement serial.device I/O

[tschak909]

PLATOTerm needs to be able to talk to RS-232 devices courtesy of serial.device (or equivalent).

To help, I've been referencing the following code: http://aminet.net/package/comm/term/MicroTerm

And of course, the RKM: http://amigadev.elowar.com/read/ADCD_2.1/Devices_Manual_guide/node009A.html

The relevant bits of code are mostly in src/io.c

in particular:

• io_init() - needs to initialize the serial device, is called when program starts.
• io_done() - needs to close down the serial device, is called when program exits
• io_send_byte() - called to send one byte to the serial port, and wait for it to do so.
• io_main() - called by main in a loop to capture any incoming serial data, and send it off to ShowPLATO.

[tschak909]

Completed roughly a week after filing this, just forgot to update ticket. Code is now in src/io.c Thanks to @wschaub for finding a very subtle error which prevented it from working.