master/output: Allow specification of per-link baud rates

Any --master or --output options may now contain a comma-separated
baud rate, e.g. --output=/dev/ttyUSB0,57600 .  This allows the use
of serial modems as output devices, multiple serial master links
with different radios etc.

If no baud is specified then the link defaults to opts.baudrate
so is backwards-compatible.