Add support for "Impassable" flag

[QuintillusCFC]

While testing movement in WWII in the Pacific, I noticed that units could move infinitely on desert. After enough testing to determine this was probably not just a bug, I checked the BIQ, and found that desert has 0 movement cost, but is impassable. Which explained the C7 behavior.

Eventually we should add impassable by wheeled too, but just plain impassable is where to start.