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/saveload/afterload.cpp

@@ -3224,6 +3224,10 @@ bool AfterLoadGame()
 		for (Station *st : Station::Iterate()) UpdateStationAcceptance(st, false);
 	}
 
+	if (IsSavegameVersionBefore(SLV_SAVEGAME_ID)) {
+		GenerateSavegameId();
+	}
+
 	if (IsSavegameVersionBefore(SLV_AI_START_DATE)) {
 		/* For older savegames, we don't now the actual interval; so set it to the newgame value. */
 		_settings_game.difficulty.competitors_interval = _settings_newgame.difficulty.competitors_interval;