mgcp: Move the "codec" params to a struct

We might be offered multiple codecs by the remote and need to
switch between them once we receive data. Do this by moving it
to a struct so we can separate between proposed and current
codec. In SDP we can have multiple codecs but a global ptime.
The current code doesn't separate that clearly instead we write
it to the main codec.
diff --git a/openbsc/contrib/testconv/testconv_main.c b/openbsc/contrib/testconv/testconv_main.c
index 773be26..6c95c55 100644
--- a/openbsc/contrib/testconv/testconv_main.c
+++ b/openbsc/contrib/testconv/testconv_main.c
@@ -64,16 +64,16 @@
 	if (argc <= 2)
 		errx(1, "Usage: {gsm|g729|pcma|l16} {gsm|g729|pcma|l16} [SPP]");
 
-	if ((src_end->payload_type = audio_name_to_type(argv[1])) == -1)
+	if ((src_end->codec.payload_type = audio_name_to_type(argv[1])) == -1)
 		errx(1, "invalid input format '%s'", argv[1]);
-	if ((dst_end->payload_type = audio_name_to_type(argv[2])) == -1)
+	if ((dst_end->codec.payload_type = audio_name_to_type(argv[2])) == -1)
 		errx(1, "invalid output format '%s'", argv[2]);
 	if (argc > 3)
 		out_samples = atoi(argv[3]);
 
 	if (out_samples) {
-		dst_end->frame_duration_den = dst_end->rate;
-		dst_end->frame_duration_num = out_samples;
+		dst_end->codec.frame_duration_den = dst_end->codec.rate;
+		dst_end->codec.frame_duration_num = out_samples;
 		dst_end->frames_per_packet = 1;
 	}
 
@@ -87,7 +87,7 @@
 	in_size = mgcp_transcoding_get_frame_size(state, in_samples, 0);
 	OSMO_ASSERT(sizeof(buf) >= in_size + 12);
 
-	buf[1] = src_end->payload_type;
+	buf[1] = src_end->codec.payload_type;
 	*(uint16_t*)(buf+2) = htons(1);
 	*(uint32_t*)(buf+4) = htonl(0);
 	*(uint32_t*)(buf+8) = htonl(0xaabbccdd);