Implement satellite event triggers through remote control.

This commit adds two new gqrx specific commands to the TCP control
interface and connects them to audio recording triggers.

The AOS command means acquistion of signal and will start the audio
recorder (unless it's already running).

The LOS command means loss of signals and will stop an ongoing audio
recording. It will do nothing if no audio recording is in progress.

These two event triggers are being added to gpredict and should be
available shortly.