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/misc.cpp

@@ -31,9 +31,11 @@
 #include "town_kdtree.h"
 #include "viewport_kdtree.h"
 #include "newgrf_profiling.h"
+#include "3rdparty/md5/md5.h"
 
 #include "safeguards.h"
 
+std::string _savegame_id; ///< Unique ID of the current savegame.
 
 extern TileIndex _cur_tileloop_tile;
 extern void MakeNewgameSettingsLive();
@@ -56,6 +58,40 @@ void InitializeCheats();
 void InitializeNPF();
 void InitializeOldNames();
 
+/**
+ * Generate an unique ID.
+ *
+ * It isn't as much of an unique ID as we would like, but our random generator
+ * can only produce 32bit random numbers.
+ * That is why we combine InteractiveRandom with the current (steady) clock.
+ * The first to add a bit of randomness, the second to ensure you can't get
+ * the same unique ID when you run it twice from the same state at different
+ * times.
+ *
+ * This makes it unlikely that two users generate the same ID for different
+ * subjects. But as this is not an UUID, so it can't be ruled out either.
+ */
+std::string GenerateUid(std::string_view subject)
+{
+	auto current_time = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::steady_clock::now().time_since_epoch()).count();
+	std::string coding_string = fmt::format("{}{}{}", InteractiveRandom(), current_time, subject);
+
+	Md5 checksum;
+	uint8 digest[16];
+	checksum.Append(coding_string.c_str(), coding_string.length());
+	checksum.Finish(digest);
+
+	return MD5SumToString(digest);
+}
+
+/**
+ * Generate an unique savegame ID.
+ */
+void GenerateSavegameId()
+{
+	_savegame_id = GenerateUid("OpenTTD Savegame ID");
+}
+
 void InitializeGame(uint size_x, uint size_y, bool reset_date, bool reset_settings)
 {
 	/* Make sure there isn't any window that can influence anything