client: collapse codecs[] and ptmap[]; allow codec variants

codecs[] is an array of enum osmo_mgcp_codecs.
ptmap[] is an array of { enum osmo_mgcp_codecs, unsigned int ptmap }.

MGCP lists first a bunch of payload type numbers and then specifies them
again for details, like the numbers 112, 96, 3 in this example:

 m=audio <port> RTP/AVP 112 96 3
 a=rtpmap:112 AMR/8000
 a=rtpmap:96 VND.3GPP.IUFP/16000
 a=rtpmap:3 GSM-FR/8000

So far we keep these lists in two separate arrays, in both struct
mgcp_response and struct mgcp_msg:
- codecs[], codecs_len stores the 'm=audio' list
- ptmap[], ptmap_len stores the 'a=rtpmap' list

These are semantically identical, and the separation means we cannot
have the same codec twice, like AMR with different fmtp variations. It
also leads to checks, conversions and dear hopes of correct ordering.

So let's keep only one list with both codec and payload type number in
it. The 'm=audio' list establishes the order of the pt numbers, and the
'a=rtpmap' list adds codec information to the established entries.

Related: OS#6171
Change-Id: I798e02c6663376d3d52f4a74fc4b32411ce95bed
---
M include/osmocom/mgcp_client/mgcp_client.h
M include/osmocom/mgcp_client/mgcp_client_fsm.h
M src/libosmo-mgcp-client/mgcp_client.c
M src/libosmo-mgcp-client/mgcp_client_fsm.c
M tests/mgcp_client/mgcp_client_test.c
5 files changed, 107 insertions(+), 68 deletions(-)