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

src/irc/iid.hpp

@@ -57,6 +57,10 @@ public:
   std::string get_sep() const;
 
 private:
+
+  void init(const std::string& iid);
+  void init_with_fixed_server(const std::string& iid, const std::string& hostname);
+
   std::string local;
   std::string server;