Add support for CompuServe B+ Protocol to SEXYZ

The Compuserve B+ protocol is supported by many 8 and 16-bit computer terminals, and is better than XModem (but appears worse than ZModem or YModem-G).

Having it available would help with retroclients.

I've uploaded the spec here: http://www.bbsdev.net/docs/bplus.txt

changed title from Add support for CompuServe B+ Protocol to SEXYX to Add support for CompuServe B+ Protocol to SEXYZ