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/os/windows/survey_win.cpp

@@ -0,0 +1,37 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file survey_win.cpp Windows implementation of OS-specific survey information. */
+
+#ifdef WITH_NLOHMANN_JSON
+
+#include "../../stdafx.h"
+
+#include "../../3rdparty/fmt/format.h"
+
+#include <nlohmann/json.hpp>
+#include <windows.h>
+
+#include "../../safeguards.h"
+
+void SurveyOS(nlohmann::json &json)
+{
+	_OSVERSIONINFOA os;
+	os.dwOSVersionInfoSize = sizeof(os);
+	GetVersionExA(&os);
+
+	json["os"] = "Windows";
+	json["release"] = fmt::format("{}.{}.{} ({})", os.dwMajorVersion, os.dwMinorVersion, os.dwBuildNumber, os.szCSDVersion);
+
+	MEMORYSTATUSEX status;
+	status.dwLength = sizeof(status);
+	GlobalMemoryStatusEx(&status);
+
+	json["memory"] = status.ullTotalPhys;
+}
+
+#endif /* WITH_NLOHMANN_JSON */