client: add unified function to generate MGCP messages

currently the only way to generate MGCP messages is to use
mgcp_msg_crcx(), mgcp_msg_mdcx() and mgcp_msg_dlcx(). All
three function take a fixed set of parameters via their
parameter list. There is no way to add or leave away optional
parameters.

add function mgcp_msg_gen(), this function takes a unified
message struct. The struct features a presence bitmask which
allows to enable and disable parameters as needed. It is also
possible to add new parameters in the future without breaking
the API.

Depends: libosmocore I15e1af68616309555d0ed9ac5da027c9833d42e3

Change-Id: I29c5e2fb972896faeb771ba040f015592487fcbe
5 files changed