Add function to display wave form from input/loop

Use 'w' to toggle display.

Move terminal input processing and main loop to main_common.c

src/common/call.h

@@ -1,7 +1,7 @@
 
 int call_init(const char *station_id, const char *sounddev, int samplerate, int latency, int dial_digits, int loopback);
 void call_cleanup(void);
-int process_call(void);
+void process_call(int c);
 
 /* received messages */
 int call_in_setup(int callref, const char *callerid, const char *dialing);