mp: Reset authentication flag after disconnecting

Otherwise it's possible to join once as moderator so that the MP mod
options are enabled, disconnect, reconnect again as non-moderator, and
still see the MP mod options enabled.

Yay for ugly hacks.

src/game_initialization/multiplayer.cpp

@@ -913,6 +913,8 @@ void start_client(game_display& disp, const config& game_config,
 	// even if the user changes the contents of the addon directory in the meantime.
 
 	DBG_MP << "starting client" << std::endl;
+
+	preferences::admin_authentication_reset r;
 	const network::manager net_manager(1,1);
 
 	gamechat.clear_history();

src/game_preferences.cpp

@@ -194,6 +194,11 @@ void parse_admin_authentication(const std::string& sender, const std::string& me
 	}
 }
 
+admin_authentication_reset::~admin_authentication_reset()
+{
+	authenticated = false;
+}
+
 static void load_acquaintances() {
 	if(!acquaintances_initialized) {
 		acquaintances.clear();

src/game_preferences.hpp

@@ -42,6 +42,17 @@ class acquaintance;
 	bool is_authenticated();
 	void parse_admin_authentication(const std::string& sender, const std::string& message);
 
+	/**
+	 * Used to reset is_authenticated flag after disconnecting.
+	 *
+	 * @todo FIXME: is_authenticated shouldn't be a preferences function. Also,
+	 *              the name is misleading.
+	 */
+	struct admin_authentication_reset
+	{
+		~admin_authentication_reset();
+	};
+
 	bool parse_should_show_lobby_join(const std::string& sender, const std::string& message);
 	int lobby_joins();
 	void _set_lobby_joins(int show);