sneaking

As suggested by qwrwed.

This reverts commit 4e5760a5.

Most modders would otherwise forget to explicitly define this, and generated nodes aliased from mods would wall-off caves

On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.

Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)

When first entering an area, sometimes placing nodes replaces other nodes that are not buildable_to. This seems to be caused by the fact that nodes in unloaded map blocks are treated as ignore, a node that is buildable_to. This fixes that, by using get_node_or_nil() instead of the previously-used get_node(), then checking to see if the nodes were actually loaded before replacing.

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.

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