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/display_wave.h

@@ -0,0 +1,12 @@
+typedef struct sender sender_t;
+
+typedef struct display_wave {
+	int	interval_pos;
+	int	interval_max;
+	int16_t buffer[256];
+} dispwav_t;
+
+void display_wave_init(sender_t *sender, int samplerate);
+void display_wave_on(int on);
+void display_wave(sender_t *sender, int16_t *samples, int length);
+