Make open trapdoor climbable

When a trapdoor is mounted upside down, to make its top surface flush
with the floor above when closed, it is necessary to have some way to
climb through the trapdoor node when it's open.  Making it climbable
like a ladder satisfies this need.  It is somewhat realistic, as a real
trapdoor can have a ladder segment mounted on one face.  When the trapdoor
is mounted in its default orientation, making the bottom surface flush
with the ceiling below when closed, the climbability when open is not
strictly necessary, but is still a convenience.