Drop nodes as items when dugged and no room in inventory and dont remove dropped items when no room in inventory

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