external ip guessing
To facilitate file transfer, external ip must be known.
To date, this is done by calling a script on goffi.org, it is explained in the README file, but this is a privacy issue (that mean that goffi.org can know the IP of people using SàT) which must be changed.
The proper method would be to use XEP-0279, but it is not always available.
So a good method must be used to respect privacy and keep file transfer easy as much as possible.
so improvment must be done in 2 times:
- first implement and use XEP-0279, and use external ip guessing as fallback. Make it possible to disable it before first launch in sat.conf
- in a second time, display a message if XEP-0279 is not available which ask user permission to do a request on goffi.org.