We were cleverly attempting to use an airlike node as the
top half of the doors, but as airlike nodes are not walkable,
falling nodes would not stop falling and thus remain an entity
stuck on top of a door.

After inspecting the builtin/game/falling.lua code, I considered
the remaining options: (a) revert doors such that the top part is
actually the door, (b) play with nodedef fields and see if other
flags may work, or (c) modify the hidden door part to another
drawtype that properly prevents this issue.

(a) seemed way over the top for now, although it would solve the
issue, it would cause a rewrite of most of the code including the
old-door-conversion.

(b) turned up nothing.

(c) turned out to be relatively simple.

So, here's the implementation where I turn the hidden door top
into a tiny, non-targetable but walkable nodebox that is entirely
inside the door hinge. It's entirely transparent, so you still
can't see it, can't hit it, nor can you place anything in it or
make liquids flow through it. The top part is placed in the right
position on placement and not touched further.

Falling nodes will properly stop on top of these doors. I've
adjusted the door conversion code to properly account for the
issue as well, so the only thing remaining is people who have
been running a git branch - those can upgrade by re-placing the
door.