Various style cleanups + unused code removal

-> Don't pass pointer to whole IGameDef to NodeMetadata constructors
	and deserializers, but only to IItemDefManager, which is needed
-> Remove the unused content_mapnode_get_new_name() method
-> Fix style for MapBlock::deSerialize and MapBlock::deSerialize_pre22,
	improving accuracy of error messages a bit
-> Fix style at other serialisation methods too
-> Improve accuracy of some comments

src/rollback_interface.cpp

@@ -155,7 +155,7 @@ bool RollbackAction::applyRevert(Map *map, InventoryManager *imgr, IGameDef *gam
 				} else {
 					NodeMetadata *meta = map->getNodeMetadata(p);
 					if (!meta) {
-						meta = new NodeMetadata(gamedef);
+						meta = new NodeMetadata(gamedef->idef());
 						if (!map->setNodeMetadata(p, meta)) {
 							delete meta;
 							infostream << "RollbackAction::applyRevert(): "