Clean up and tweak build system

* Combine client and server man pages.
  * Update unit test options and available databases in man page.
  * Add `--worldname` to man page.
  * Fix a bunch of places where `"Minetest"` was used directly instead of `PROJECT_NAME`.
  * Disable server build by default on all operating systems.
  * Make `ENABLE_FREETYPE` not fail if FreeType isn't found.
  * Enable LevelDB, Redis, and FreeType detection by default.
  * Remove the `VERSION_PATCH_ORIG` hack.
  * Add option to search for and use system JSONCPP.
  * Remove broken LuaJIT version detection.
  * Rename `DISABLE_LUAJIT` to `ENABLE_LUAJIT`.
  * Rename `minetest_*` variables in `version.{h,cpp}` to `g_*`.
  * Clean up style of CMake files.

src/script/lua_api/l_mainmenu.cpp

@@ -980,7 +980,7 @@ int ModApiMainMenu::l_show_file_open_dialog(lua_State *L)
 /******************************************************************************/
 int ModApiMainMenu::l_get_version(lua_State *L)
 {
-	lua_pushstring(L, minetest_version_simple);
+	lua_pushstring(L, g_version_string);
 	return 1;
 }