Settings: Proper priority hierarchy

Remove old defaults system
Introduce priority-based fallback list
Use new functions for map_meta special functions
Change groups to use end tags

Unittest changes:
 * Adapt unittest to the new code
 * Compare Settings objects

src/map_settings_manager.h

@@ -44,8 +44,7 @@ struct MapgenParams;
 */
 class MapSettingsManager {
 public:
-	MapSettingsManager(Settings *user_settings,
-		const std::string &map_meta_path);
+	MapSettingsManager(const std::string &map_meta_path);
 	~MapSettingsManager();
 
 	// Finalized map generation parameters
@@ -71,6 +70,6 @@ class MapSettingsManager {
 
 private:
 	std::string m_map_meta_path;
+	// TODO: Rename to "m_settings"
 	Settings *m_map_settings;
-	Settings *m_user_settings;
 };