Make guarding streets a bit better

Guarding cars are faster. Hero moves a bit slower on streets. Also add "guard"
attrobute for .xml placement file. A car can be made into a guarding/agressive
car with guard="nonemptystring"

Other fixes:

- Also fixed layers for maps/cities.
- Starfish now disappear when hero finds them in the city.
- Render generic layer numbers, not only 1 or 2 layers.