Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

src/settings_type.h

@@ -63,13 +63,20 @@ enum IndustryDensity {
 	ID_END,       ///< Number of industry density settings.
 };
 
-/** Possible values for "userelayservice" setting. */
+/** Possible values for "use_relay_service" setting. */
 enum UseRelayService {
 	URS_NEVER = 0,
 	URS_ASK,
 	URS_ALLOW,
 };
 
+/** Possible values for "participate_survey" setting. */
+enum ParticipateSurvey {
+	PS_ASK = 0,
+	PS_NO,
+	PS_YES,
+};
+
 /** Settings related to the difficulty of the game */
 struct DifficultySettings {
 	byte   competitor_start_time;            ///< Unused value, used to load old savegames.
@@ -306,7 +313,8 @@ 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?
+	UseRelayService use_relay_service;                    ///< Use relay service?
+	ParticipateSurvey participate_survey;                 ///< Participate in the automated survey
 };
 
 /** Settings related to the creation of games. */