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/bnetz/dsp.h

@@ -1,6 +1,6 @@
 
 void dsp_init(void);
-int dsp_init_sender(bnetz_t *bnetz);
+int dsp_init_sender(bnetz_t *bnetz, double squelch_db);
 void dsp_cleanup_sender(bnetz_t *bnetz);
 void bnetz_set_dsp_mode(bnetz_t *bnetz, enum dsp_mode mode);