Check if the SDP file has any codec potentially supported by GSM.
The topic of codec selection is a complicated one and we will not
support it correctly in the beginning.
* Create a new handle
* Send the invite
* Have some state transitions
* Allow to release a call in initial unconfirmed state, confirmed
one with cancel and connected with bye
* Add simple SDP parsing to find the rtpmap/codec that is used by
gsm
This code is capable of creating an agent that will bind on the
configured local address. The next steps are to configure the
library in terms of allowed features and prepare call handling.
Holger Hans Peter Freyther