This makes using STUN mandatory for games hosted using the multiplayer
lobby. The motivation for that is a reduction in complexity, because
right now if STUN is disabled we use a home-grown STUN-like logic, which
got implemented before Pyrogenesis got STUN support.

That home-grown logic relies on a custom ejabberd module (mod_ipstamp),
which inserts the external IP-address of a host in the response messages
when a host registers a game. Originally mod_ipstamp was also used to
inform all potential players of a hosts IP-address, however that has
already been removed to let hosts to only share their IP-address with
players actually joining their game.

Removing the home-grown logic and instead always relying on STUN removes
complexity in Pyrogenesis and the lobby server and also eases hosting
games for players, as they don't have to figure out anymore whether they
need to enable STUN or not.

These changes shouldn't negatively impact the ability of Pyrogenesis to
handle different types of NAT or broken networks. There is one
difference though: While the custom logic using mod_ipstamp utilized TCP
as transport protocol, the STUN implementation in Pyrogenesis currently
uses UDP. That doesn't allow hosts with UDP-connectivity issues to
resolve their external IP-address anymore, however without
UDP-connectivity they aren't able to successfully host games anyway, as
the actual game updates are transferred using UDP as well.

TODO:
- remove lobby.stun.enabled config option

@@ -229,3 +228,3 @@
 	CStr ip;
 	u16 port = 0;
-	if (g_XmppClient && m_UseSTUN)
+	if (g_XmppClient && !localNetwork)