Also fix some constness things

Remove the close() method and closed attribute. Remove all the calls to
close().  (Save one bool per XmlNode, yay, and save a few ifs and some
useless function calls. At best it should be unnoticeably faster and lighter
and save a few keystrokes in the future)

They were not able to execute them anyway, so this was just a little
usability issue.

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

Only keep some biboumi-specific commands into biboumi_adhoc_commands.hpp/cpp