add simplistic IuUP FSM and strip/add IuUP headers

This should really be using the FSM in libosmocore/laforge/iu_up: take the best
of both sides and integate in the libosmocore FSM implementation, then use it
here.
- in libosmocore, the FSM definition is nicer.
- here, we have correct header checksums.

Introduce using msgb to receive, pass and send RTP packets.

Add/strip IuUP from RTP data (for which msgb is particularly useful).

The payload type on an IuUP conn is maintained as negotiated in the IuUP
Initialization. For the pure RTP side, an SDP "AMR" ptmap attribute is looked
up, so that payload type numbers are translated between IuUP <-> RTP.

Change-Id: Ibc70e0aa00476926dd1f4ea8139c34f31f9cdfa3
diff --git a/include/osmocom/mgcp/mgcp_codec.h b/include/osmocom/mgcp/mgcp_codec.h
index 3ead60a..caeecb0 100644
--- a/include/osmocom/mgcp/mgcp_codec.h
+++ b/include/osmocom/mgcp/mgcp_codec.h
@@ -5,3 +5,5 @@
 int mgcp_codec_add(struct mgcp_conn_rtp *conn, int payload_type, const char *audio_name, const struct mgcp_codec_param *param);
 int mgcp_codec_decide(struct mgcp_conn_rtp *conn);
 int mgcp_codec_pt_translate(struct mgcp_conn_rtp *conn_src, struct mgcp_conn_rtp *conn_dst, int payload_type);
+const struct mgcp_rtp_codec *mgcp_codec_pt_find_by_subtype_name(struct mgcp_conn_rtp *conn,
+								const char *subtype_name, unsigned int match_nr);