Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.

The items will now just disappear when dropped.

This allows the removal of nodes with unknown types.
get_item_callback() (C++) would fail if a node has an unknown type.  Now it
will try using the callback from minetest.nodedef_default in this case.
Also, minetest.node_dig() (Lua) was altered to always allow digging when
the node definition is empty (i.e. unknown node).

builtin/item.lua: callbacks with copies of positions and nodes rather than recycle the same ones, which callbacks can modify

Remove tiles and special_tiles from node definition prototype because otherwise the old names can't be used