Because apparently when cloning a repository with submodules, git creates
empty sub-directories

ref #2472

in this order:
- Make sure it is utf-8 encoded
- Remove all chars that are invalid in XML
- Escape all XML special chars (&'"<>)

By using SYSTEMD_FOUND instead of SYSTEMDDAEMON_FOUND, where I forgot to
rename it…

Also fix some doc

This option lets the administrator choose a specific IRC server, and only
that server can be used with this biboumi instance.

In this mode, JIDs to use are changed like this:

- #chan%irc.example.com@biboumi.example.com      -> #chan@biboumi.example.com
- user!irc.example.com@biboumi.example.com       -> user!@biboumi.example.com
- #chan%irc.example.com@biboumi.example.com/Nick -> #chan@biboumi.example.com/Nick
- %irc.example.com@biboumi.example.com           -> no equivalent
- irc.example.com@biboumi.example.com            -> no equivalent

fix #2715

fix #2533

When the connection is lost, immediately try to reconnect, then try to
reconnect every 2 seconds.  This is much better than the previous “Try to
re-connect as fast as possible”.

fix #2946

Because we need to wait for the welcome message, when we connect to the
server, before sending the JOIN command, we need to also save the value of
the password to reuse it when we actually send the JOIN command

fix #2882