Convert stun code to gio

* Remove stun_conn->test, as it's never set to anything other than 1.
* De-static stun_conn->packet.
* Use GSocket in stun code.

Testing Done:
Set stun server to stun.ekiga.net (gives my IP), then also broken address (doesn't give my IP). Seems to work without crashing or hanging.

Reviewed at https://reviews.imfreedom.org/r/227/