Rework on audio buffer management

Use function to get samples to be sent to fill audio buffers to a
level. This replaces the function that only shows how much data
is in the buffer.

This way the function itself can control how much data will be
sent.

src/common/sender.h

@@ -46,7 +46,7 @@ typedef struct sender {
 	void 			(*audio_close)(void *);
 	int			(*audio_write)(void *, sample_t **, int, enum paging_signal *, int *, int);
 	int			(*audio_read)(void *, sample_t **, int, int);
-	int			(*audio_get_inbuffer)(void *);
+	int			(*audio_get_tosend)(void *, int);
 	int			samplerate;
 	samplerate_t		srstate;		/* sample rate conversion state */
 	double			rx_gain;		/* factor of level to apply on rx samples */