Update our OSM export.

We're actually now *ahead* of OSM for a while, since as I write this the
OSM API is down and so I can't upload my latest changes.
Treat steps as footway, but add an extra tag so we can render differently.

No different rendering yet, though.
Only highlight public transit roads that are walkable.

It's not interesting to know a bus runs on a street unless you can walk
along the street and catch it.
Reduce footway smoothing for less-crazy rendering.

The full smoothing was causing some of the zig-zag footways in SF to
render as wobbly lines instead.
Set the map's bounds to where there's currently something to render.
Only apply default sidewalks if foot!="no".

Sometimes people say foot=no instead of sidewalk=none, so we need to
handle that case so we don't send people walking down inappropriate roads.