This stores three flags in unused map bits, and avoids having to look up
station graphics and custom station specs to determine blocked/wires/pylons
status.
This potentially affects rail pathfinding performance.
Savegame version is not bumped, as the flags can just be updated every
time.
(cherry picked from commit cb658e6cc6e1c114c1eaafaaa5b14849f04ca083)