Fix buses trying to warp between lanes between stops

[dabreegster]

#372

This situation was previously happening: a bus stop is on the rightmost lane of a road, and the path to the next bus stop immediately requires a turn from the leftmost lane. Before some extra validation in the previous PR, the bus would just warp in the simulation and crash things.

But really the fix is to force pathfinding from that exact start lane when it's a bus. (Most vehicle drives exit from a driveway and are allowed to immediately enter any starting lane.) This PR probably does that. The one test case I was looking at in SP actually isn't routable with this new constraint, because there's a turn restriction at https://www.openstreetmap.org/node/2389060511.

I need to regenerate maps and prebaked data and validate still.