(including 1200/2200Hz) to -x command line argument in Direwolf.
This is useful for calibrating FM deviation using a meter,
devcal, or manually. See:
https://groups.io/g/direwolf/topic/setting_deviation/78633292
Options are:
-x a: alternating mark/space tones (original functionality)
-x m or -x 1: steady mark tone (e.g. 1200Hz)
-x s or -x 2: steady space tone (e.g. 2200Hz)
-x p: set PTT only - for RTL-SDR adjustments
Scope tested with mark_freq>=300Hz and space_freq<=5000Hz.
Returns an error message if mark/space frequencies are set
to 0.