Improve placing attached nodes at side of buildable_to nodes

[Wuzzy2]

Currently, placing a attached or wallmounted nodes (torches, signs, …) at the side of a buildable_to node (e.g. tall grass) fails fairly often.
It only works if there's a solid node at the other side. This can be annoying sometimes.

I want to change the behaviour:
When placing at the side of a buildable_to node, attempt to place the attached node on the floor instead.