common code: Add feature to record received audio and replay it

-W <file.wav> writes a wave file of received audio
-R <file.wav> reads a wave file to feed into decoder

This way you can record a phone and later debug without a phone
and radio equipment.

src/common/main.h

@@ -8,6 +8,8 @@ extern int use_mncc_sock;
 extern int send_patterns;
 extern int loopback;
 extern int rt_prio;
+extern const char *read_wave;
+extern const char *write_wave;
 
 void print_help(const char *arg0);
 void print_help_common(const char *arg0, const char *ext_usage);