Implementation of RX level squelch (for A-Netz and B-Netz)

Use -S <dB> for setting RF level or use -S auto for auto level.

When squelch closes, audio is muted. If squelch is closed for some
seconds (depending on network), call is released. (RF loss condition)

The previous loss detection has been removed

src/cnetz/dsp.c

@@ -550,7 +550,7 @@ static int fsk_distributed_encode(cnetz_t *cnetz, const char *bits)
 /* decode samples and hut for bit changes
  * use deviation to find greatest slope of the signal (bit change)
  */
-void sender_receive(sender_t *sender, sample_t *samples, int length)
+void sender_receive(sender_t *sender, sample_t *samples, int length, double __attribute__((unused)) rf_level_db)
 {
 	cnetz_t *cnetz = (cnetz_t *) sender;