[C128] Userport serial driver

[rhalkyard]

At present, the only supported serial interface for the C128 version is the SwiftLink UART cartridge. This is fine (preferable, even) for running in emulation, but on real hardware, support for software-driven RS232 over the user port (like in the UP2400 driver for the C64) would be desirable, as it doesn't require any additional hardware.

While the UP2400 driver is external to the PlatoTerm project, writing a similar driver for the C128 would be beneficial to the project, as it would allow C128 users without SwiftLink cartridges (such as myself) to use the high-res C128 PlatoTerm instead of the C64 version.

The magazine article that the UP2400 driver was based on ("Towards 2400", Transactor Magazine, vol. 9, issue 3, pp. 62) appears to talk about support for the 128 and even 2MHz mode, so in theory at least, it should be possible to modify the existing UP2400 driver code for use on the 128.