allow elevator to build into any buildable node

[fluxionary]

Allow the elevator to extend into any space that contains a node which is buildable_to (i.e. not just air), like the travelnet.

Sorry, but my IDE also cleaned up some trailing whitespace, so a lot more lines look modified than actually are.

[thomasrudin]

Merged here: https://github.com/mt-mods/travelnet (@Sokomine please tell me when you are maintaining this again)