Refactored vehicle spawning

* Pulls the four copies of "spawn city wrecks / out of fuel vehicles" into a single function. No more desync!
* Car chassis have officially died in a fire and will only be found in garages from now on. Closes #3129. In their place:
  * Cars are more likely to spawn
  * Slight chance of finding a beetle, scooter, or motorcycle with sidecart on the roads now.

Also cuts down the size of mapgen.cpp a little, but it's kind of like throwing a deckchair off the Titanic.