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/tv/main.c

@@ -324,7 +324,7 @@ static void tx_bas(sample_t *sample_bas, __attribute__((__unused__)) sample_t *s
 		int s, ss, tosend;
 		while (!quit) {
 			usleep(1000);
-			sdr_read(sdr, (void *)sendbuff, latspl, 0);
+			sdr_read(sdr, (void *)sendbuff, latspl, 0, NULL);
 			tosend = sdr_get_tosend(sdr, latspl);
 			if (tosend > latspl / 10)
 				tosend = latspl / 10;