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

Scripts can call minetest.register_on_shutdown() to register a
shutdown hook.

Document that minetest.register_on_shutdown() callbacks may not be run

If the server crashes, it is unlikely that callbacks registered using
minetest.register_on_shutdown() will be called.

This reverts commit ffad18e4.

According to #253, using `/clearpassword` without an argument causes the server to crash from an assertion failure. I've resubmitted matttpt's patch as a pull request to aid in merging.

The items will now just disappear when dropped.

Commands moved:
 /me
 /status
 /time
 /shutdown
 /ban
 /clearobjects

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