Clean up ModeSDemodulator dummy audio out.

* Use a mono output now that we can.
* Use set_max_output_buffer to make throttle output smoother.
Use gr-air-modes instead of dump1090.

gr-air-modes will be more suitable for further interpretation of the
output. dump1090 has two flaws for this purpose: it has no output format
really suitable for parsing, and it does not flush stdout promptly.