#117 export extract_mediaip feature from nathelper.c

[Anonymous]

allowing to use something like

$avp(s:mediaip) = extract_mediaip()

The feature would be useful when messages are passing through the same proxy twice (once in, once out) to store the ip of the media in the way in, and should the message out go back to the same ip, restore the IP in order to not route the rdp stream via the proxy.