Param2 prediction for wallmounted nodes

Placing torches and other wallmounted nodes looks really laggy, even in singleplayer mode, because the node placement prediction code sets param2 to zero, aka hanging from the ceiling, which few torches are.

This patch makes it so the correct param2 value is predicted, as long as the server uses the builtin item_place or a compatible function. It also moves node placement prediction to a separate function, which makes the_game a minuscule bit shorter. Third, it makes the server send the MapBlock corresponding to pointed_pos_under after a node with prediction has been placed (necessary when placing to a buildable_to node on a block boundary).

Prediction for facedir or attached_node is not included, this can be done later if desired.