(RFC) Make monster density depend on nearby buildings

See #350

Currently, mapgen won't place zombies in most overmap tiles, including some roads, which stops zombies from spawning outside the edge of town, when we probably want a smooth drop-off of zombies. I don't think this can be easily changed, because some overmap tiles spawn zombies in different ratios to normal using different monster groups, but maybe I'm missing something.

The numbers used for mondensity will probably need to be changed from their current values.