Fix U to accept UA..UI as well as U1..U9
Add U9/UI for firmware detection

UI copies the version string into the status channel.
IEC: handshake LISTEN/TALK only if it's for us

This enables checking if a device is present
and makes drive toggle with JiffyDOS / Ctrl-D work.
Fixes #56
Fix XS-1541 / 3.1541 firmware update scripts
Merge branch 'master' of github.com:fachat/XD2031 into filetypes