Feature: allow the use of TURN to connect client and server together

TURN is a last resort, used only if all other methods failed.
TURN is a relay approach to connect client and server together, where
openttd.org (by default) is the middleman.

It is very unlikely either the client or server cannot connect to
the STUN server, as they are both already connected to the Game
Coordinator. But in the odd case it does fail, estabilishing the
connection fails without any further possibility to recover.

src/settings_type.h

@@ -59,6 +59,13 @@ enum IndustryDensity {
 	ID_END,       ///< Number of industry density settings.
 };
 
+/** Possible values for "userelayservice" setting. */
+enum UseRelayService {
+	URS_NEVER = 0,
+	URS_ASK,
+	URS_ALLOW,
+};
+
 /** Settings related to the difficulty of the game */
 struct DifficultySettings {
 	byte   competitor_start_time;            ///< Unused value, used to load old savegames.
@@ -290,6 +297,7 @@ struct NetworkSettings {
 	bool        reload_cfg;                               ///< reload the config file before restarting
 	std::string last_joined;                              ///< Last joined server
 	bool        no_http_content_downloads;                ///< do not do content downloads over HTTP
+	UseRelayService use_relay_service;                        ///< Use relay service?
 };
 
 /** Settings related to the creation of games. */