Add a list of permanent channels, on each IRC-server

Add an option (of type “string-list”) on each IRC server, to maintain a list of channels that should not be left (do not send a PART message to the channel) when the last resources leaves it.

Also, an option (boolean) to automatically add channel’s names to this list whenever a new channel is joined.

This is useful to make biboumi act like a “bouncer”, in conjunction with #3229 (closed).

The only way to leave these channels would be to manually send a PART message to them, using a raw command, or to disconnect from the server.