Add support for symmetric RTP/RTCP

[wmanley]

Symmetric RTP/RTCP makes it easier for RTP/RTCP sessions to work through Cone NAT as used in many home routers by reusing the same UDP port for receiving both RTP and RTCP messages which in turn is the same port that is used to send outgoing RTCP messages to the re-transmission server.

vqe-c has support for STUN and UPNP for NAT traversal, symmetric RTP has the advantage that it doesn't require any additional servers (such as STUN servers) or home routers to have additional functionality (UPNP).

[wmanley]

The input shim seems to be a good place to start WRT these modifications. In particular functions vqec_dp_input_shim_repair_inject seems interesting from the perspective of the comment.