Per rP21520, specifically elexis' comments here: https://code.wildfiregames.com/rP21520#31215
Partially overlaps with, and invalidates D1666 (I failed to notice this diff). My diff addresses JoshuaJB's remark there.
We currently hardcode the expectation that hosts use the "/0ad" XMPP resource, which 0 A.D. uses. This is annoying for dedicated servers, and in general not particularly a good idea (weirdly, it's actually less effort to not do it).
This changes that. It's a first step towards dedicated hosting.
The host advertises their own JID because we use that for direct IP querying, to set up a game, so it's actually an essential component nowadays, and I don't believe there are any particular security concerns.
One could spoof and use somebody else's JID, but that would merely make their game a 'proxy' and we can update the lobbybots to prevent that anyways (and we probably should). This isn't done in this particular diff because I'd prefer D2630 to get merged first.