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

docs/a-netz.html

@@ -259,6 +259,9 @@ Be sure that the phone turns off the transmitter and indicates the (green) light
 
 <p>
 Level adjustment:
+</p>
+
+<p>
 We see a receive level of around 140%.
 Then start the base station using '-l 2' option for loop-back and tune receiver to the transmitter.
 The base station generates a 1750 Hz test signal, just like the mobile phone.
@@ -308,6 +311,9 @@ Be sure to check: Does your transmitter has enough frequency deviation (15 KHz i
 
 <p>
 Detecting loss of carrier signal:
+</p>
+
+<p>
 To automatically release the call, when the carrier signal gets lost, look at the B-Netz page.
 It is the same principle.
 </p>