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/server.h

@@ -524,6 +524,7 @@ private:
 	u16 m_max_chatmessage_length;
 	// For "dedicated" server list flag
 	bool m_dedicated;
+	Settings *m_game_settings = nullptr;
 
 	// Thread can set; step() will throw as ServerError
 	MutexedVariable<std::string> m_async_fatal_error;