don't remove node above door if it's not a doors:hidden node

[fluxionary]

currently, doors will remove the node above the door if the door is destroyed in various ways. however, there's times when the node above the door may not actually be a "doors:hidden" node, because of other mods. in particular, a mesecons sticky piston can be used to destroy an arbitrary node, whether or not it is private (can only be destroyed by a certain player) or protected (can only be modified if minetest.is_protected returns false).

this PR checks that the node above the door is actually a "doors:hidden" node before destroying it.

[appgurueu]

Should both of these return {name}? I believe the return {name} here should be removed, otherwise we get varying drops based on a race condition:

• If the top is blasted first, we get 2 doors.
• If the bottom is blasted first, the top is removed, and we only get one door.

Also: Am I missing something, or could you duplicate doors by blowing them up with TNT (before this patch)?

And in general, concerning the logic: Shouldn't both nodes remove each other to avoid inconsistent states? A "bottom without hidden top" shouldn't happen either, should it?

[fluxionary]

i'm deliberately not looking into whether return {name} is correct right now, or what the deal is w/ tnt. i'll try to look at it soon though. perhaps that should be a separate issue.

the door and "hidden" node can currently be separated by e.g. a mesecons piston. i really doubt anyone wants the doors mod to depend (optionally) on mesecons, and other mods could introduce that same kind of mechanic in the future, so that wouldn't even solve the problem in general. i suppose mesecons could automatically register all doors as MVPS stoppers? e.g. iterate doors.registered_doors and also override doors.register.

[appgurueu]

Definitely an improvement over removing an arbitrary node. Further improvements can be made later on.

The local above = ...; if is...(above) then remove_node(...) end is repeated three times. Perhaps rather than having is_door_upper_node, extract this into a function try_remove_upper_node?