Changes

mgcp_client: add fmtp string to struct ptmap

Allow MGCP clients to pass arbitrary fmtp strings to MGWs, beyond
the fixed set of parameters captured in legacy struct mgcp_codec_param,
and set a different fmtp string per payload type, as opposed to global.

Credit: this patch is a derivative work based on Neels Hofmeyr's
patch If58590bda8627519ff07e0b6f43aa47a274f052b from WIP branch
neels/sdp, reduced to just libosmo-mgcp-client.

Present necessity: this functional addition is needed in order to
allow osmo-bsc to pass this construct to its MGW when the CN
requested the use of TW-TS-006 extended payload format for AMR:

	a=rtpmap:112 AMR/8000/1
	a=fmtp:112 octet-align=1; tw-ts-006=1

AMR codec fmtp parameter tw-ts-006 (defined in TW-TS-006 spec
section B.1) is not supported by osmo-mgw; however, it is supported
by tw-e1abis-mgw, which is the OsmoBSC-compatible MGW needed for
E1 BTS with AMR and CSD.

Change-Id: I84ba2ed5ab9d379ac0b675520796446ad6ee0710