Mapgen cleanup round 1

The function map::draw_map is WAY too long - over 10,000 lines, at the moment. I honestly cannot parse it. It's one long switch statement, which IMO means it should be using helper functions.

Here's the first of multiple batches of changes to extract code from map::draw_map and move it into helper functions. No big rush on this, but I want to publicize what I'm doing so that other people working on mapgen see what my plans are.