opus: add opus_stereo config parameter

  opus_stereo    yes   -- Enable stereo (default)
  opus_stereo    no    -- Disable stereo

thanks to Ola Palm for the original patch

modules/opus/opus.c

@@ -36,7 +36,7 @@
 
 
 static bool opus_mirror;
-static char fmtp[256] = "stereo=1;sprop-stereo=1";
+static char fmtp[256] = "";
 static char fmtp_mirror[256];
 
 
@@ -88,9 +88,19 @@ static int module_init(void)
 	struct conf *conf = conf_cur();
 	uint32_t value;
 	char *p = fmtp + str_len(fmtp);
-	bool b;
+	bool b, stereo = true;
 	int n = 0;
 
+	conf_get_bool(conf, "opus_stereo", &stereo);
+
+	/* always set stereo parameter first */
+	n = re_snprintf(p, sizeof(fmtp) - str_len(p),
+			"stereo=%d;sprop-stereo=%d", stereo, stereo);
+	if (n <= 0)
+		return ENOMEM;
+
+	p += n;
+
 	if (0 == conf_get_u32(conf, "opus_bitrate", &value)) {
 
 		n = re_snprintf(p, sizeof(fmtp) - str_len(p),