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/table/settings/network_private_settings.ini

@@ -8,6 +8,7 @@
 
 [pre-amble]
 static constexpr std::initializer_list<const char*> _use_relay_service{"never", "ask", "allow"};
+static constexpr std::initializer_list<const char*> _participate_survey{"ask", "no", "yes"};
 
 static const SettingVariant _network_private_settings_table[] = {
 [post-amble]
@@ -16,6 +17,7 @@ static const SettingVariant _network_private_settings_table[] = {
 SDTC_BOOL  =  SDTC_BOOL(              $var,        $flags, $def,                              $str, $strhelp, $strval, $pre_cb, $post_cb, $from, $to,        $cat, $extra, $startup),
 SDTC_OMANY = SDTC_OMANY(              $var, $type, $flags, $def,             $max, $full,     $str, $strhelp, $strval, $pre_cb, $post_cb, $from, $to,        $cat, $extra, $startup),
 SDTC_SSTR  =  SDTC_SSTR(              $var, $type, $flags, $def,             $length,                                  $pre_cb, $post_cb, $from, $to,        $cat, $extra, $startup),
+SDTC_OMANY = SDTC_OMANY(              $var, $type, $flags, $def,             $max, $full,     $str, $strhelp, $strval, $pre_cb, $post_cb, $from, $to,        $cat, $extra, $startup),
 
 [validation]
 SDTC_OMANY = static_assert($max <= MAX_$type, "Maximum value for $var exceeds storage size");
@@ -90,3 +92,12 @@ str      = STR_CONFIG_SETTING_USE_RELAY_SERVICE
 strhelp  = STR_CONFIG_SETTING_USE_RELAY_SERVICE_HELPTEXT
 strval   = STR_CONFIG_SETTING_USE_RELAY_SERVICE_NEVER
 cat      = SC_BASIC
+
+[SDTC_OMANY]
+var      = network.participate_survey
+type     = SLE_UINT8
+flags    = SF_NOT_IN_SAVE | SF_NO_NETWORK_SYNC
+def      = PS_ASK
+min      = PS_ASK
+max      = PS_YES
+full     = _participate_survey