This makes the program compatible with future protocol updates.
Before it was just dying so even if the new protocol was compatible
the program needed to be recompiled.

My Videohub Micro doesn't support this extra but I suppose the
bigger models do (at least the official software v6 reads this
value and shows it instead of the device model).

--list-proc-units was documented in 1.0 release but it was never
wired up to work. --list-frames was not documented in 1.0 and it
wasn't working either.

It is not tested on a real hardware. The number of frame buffers
is assumed to be the same as number of serial ports (as good guess
as any).

It is not tested on a real hardware.