When set to true, the realname and username are extracted (by default) from
the user’s JID

fix #3136

Instead of ignoring the stanza, we send back an error of type
remote-server-not-found each time it's possible.

Also avoid having to do if (!irc) return; everytime.

fix #3045

Messages received on an IRC server JID are forwarded as raw IRC messages.

fix #2486

ref #2472

fix #2946

fix #2575

ref #2575

ref #2757

A result or an error, depending on the type of message

For example, both JID #chan%server@biboumi/Toto and toto!server@biboumi are
equivalent, except that if you send a message to the first one, subsequent
messages coming from the user toto will come from that same JID. This is
done to be consistent for the XMPP user, and respond from the same JID than
the 'to' of the first message.

fix #2468

fix #2452

We want to call socket() with the parameters provided by getaddrinfo, so we
can’t addd the fd into the poller immediately. We need to wait the
connection attempt, and then the SocketHandler can call add_socket_handler
itself, if the connection succeeds, or is in progress.

fix #2524

Also remove the duplicate send_self_join methods, user only send_user_join

The generated XML is very verbose because each IRC formatting tag makes us
close a <span/> element and reopen it with the new style applied.  However,
this works quite well and is easy to implement.