* Add support for `group:supported_node` to CORE/mcl_attached.
Supported nodes are nodes that can be placed on any node that does not
have the `drawtype = "airlike"` attribute.
* Copy the `drop_attached_node()` function from minetest/builtin, so
that the override function provides the same behavior when nodes drop.
* Add comments to CORE/mcl_attached and to the functions defined in it.
* Add more local aliases for global minetest.* functions.
* If the original function returns true, it is not necessary anymore
to perform more tests and the override function can simply return true
immediately.
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