[JDEV] presence fixes in cvs

[Jeremie]

Thanks for the patch(es), I've got the obvious bug fixed in mod_presence,
as well as another one relating to sending presence after a subscribed in
the same session.

What would be great now is if people could test current cvs, particularly
the presence part, and make sure all the logic seems to be on par with
1.4.1.  If you want to test the invisible mode features, instead of
sending presence normally when you log in, send it like so:

	<presence to="yourname at yourserver"/>

By sending it to yourself it's telling the server that you are online only
to yourself (you get offline messages, other resources are notified, etc).
It will also check your roster and probe s10ns so that you can see others.
It supports switching to invisible mode during a session (send
type=unavailable and then the invisible presence).

At any time, invisible mode or not, you can send available presence to=""
any jid and they will receive any updates thereafter, or send unavailable
and they will no longer receive any presence changes (so you can go
offline to someone in your roster persistently even if you update your
presence).

I'd appreciate any feedback on the invisible mode, bug reports, etc...
I'll try to keep up on any posts to jdev :)

Jer