Restructure: Move dtmf from common code to 'libdtmf'

src/libdtmf/dtmf_encode.h

@@ -0,0 +1,14 @@
+
+typedef struct dtmf_enc {
+	int	samplerate;		/* samplerate */
+	char	tone;			/* current tone to be played */
+	int	pos;			/* sample counter for tone */
+	int	max;			/* max number of samples for tone duration */
+	double	phaseshift65536[2];	/* how much the phase of sine wave changes per sample */
+	double	phase65536[2];		/* current phase */
+} dtmf_enc_t;
+
+void dtmf_encode_init(dtmf_enc_t *dtmf, int samplerate, double dBm_level);
+void dtmf_encode_set_tone(dtmf_enc_t *dtmf, char tone);
+void dtmf_encode(dtmf_enc_t *dtmf, sample_t *samples, int length);
+