Add squelch tool

[W3AXL]

added a squelch tool to the library, which functions similar to the resampler tool (and in fact the basic code structure was shamelessly stolen from it)

The squelch algorithm is very similar to what KA7OEI describes: my implementation detects noise above 4khz using an FIR HPF and calculates the energy of the isolated signal. This is then averaged and scaled to be within a ~1-12 integer range. Output audio is unmuted based on comparison of this calculated energy and the inverse of the set squelch level.

Features:

• Takes in a fifo of demodulated audio samples (from multifm or similar)
• Can output to another fifo or to stdout for piping into tools like ffmpeg
• Configurable silence modes - silence (continuous 0-valued samples) or truncation (no output until unsquelched)
• Supports sample rates of 12k, 16k, 20k, and 24k (limited by pre-defined FIR filters for HPF)
• Variable squelch level from 0 (open squelch) to 10 (tight)
• All calculations are fixed-point integers to save on processing power as much as possible
• All FIR filters used for noise isolation are <100 taps.

Example usage, reading FM demod samples from infifo and saving the squelched audio to a wav file using ffmpeg:

$ squelch -S 12000 -L 7 ./infifo | ffmpeg -f s16le -ar 12000 -ac 1 -i pipe:0 squelch_test.wav

The -h flag gives additional usage parameters.

Future expansion could include CTCSS tone detection using the Goetzel algorithm, and probably DCS as well without too much fuss.