Add a changelog entry for the fixed_irc_server option

Merge branch 'fixed_server'

Implement room discovery using the LIST irc command

ref #2472

Properly sanitize everything in the XML we send to the XMPP server

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

Fix the systemd-conditional code

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

Remove an unused parameter

Do not handle the "%" char in a special way, in the fixed_server mode

Also fix some doc

Add support for a fixed_irc_server configuration

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

Include the IRC hostname in the IRC RECEIVING and SENDING debug messages

fix #2715

Forge link uses https

Add a reload add-hoc command

Update the CHANGELOG and INSTALL files, mentioning c-ares

Add the FindCARES cmake module

Implement async DNS resolution using c-ares

fix #2533

Fix typo

Use a timer to try reconnecting to the XMPP server only each 2 seconds

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”.

Remove the XmppComponent::last_auth bool that was never used anywhere

Properly display [DEBUG] on debug lines, instead of [ERROR]

Fix “connections” typo

Fix “cleanely” typo