Add a Spread_type which dictates how bonus tiles will spread.
Use Spread_type in Worldmap::add_bonus().
Allow relational maps with 0 neighbors to respect the WTF_FACE_ROAD flags.  E.g. a single-tile factory will rotate to face the road; multi-tile factories will not, of course.
Add 0-neighbor maps for road/bridge, just in case.
Factories.  WIP - they won't work right now.