Better handle high sample rate audio devices.

[tmiw]

This PR improves the behavior of FreeDV when using audio devices at higher sample rates (e.g. >48000) and channel counts (e.g. the 4 channel minimum the Focusrite Scarlett has on Windows). Basically, the following in the code's changed:

1. Remove the arbitrary 48 kHz limitation in the main RX and TX processing loops.
2. Add logic to detect the minimum number of channels for an audio device (PortAudio only; currently hardcoded to 1 on PulseAudio/pipewire).
3. Remove hardcoding of the two channel limit in the audio handling callback.
4. Fix any bugs encountered during testing of the above.