Heat up bodyparts over adjacent fires

Makes the non-sleeping survivors heat up bodyparts when there's an adjacent fire. Torso, head and legs don't get much heat unless the fire is big. Arms get more and hands get a lot, because you can get them close to fire.
Feet get only a tiny bit, unless the survivor is on a chair and then they get a lot.

Only chairs, armchairs and benches are recognized as chairs. Beds would probably catch fire from the sparks, vehicle seats would probably be hard to get in a good position.
Any other good chair-type furniture? Something non-flammable would be nice, because currently comfy campfire isn't possible.

Only the best fire is used, because one cannot extend arm over two fires at once.

Additionally I fixed a moderately severe bug with fire heat: fire heat would use max(1,max(j, k)) as distance, despite j and k often having negative values. This would make fires to the north and to the west always as hot as if they were adjacent.
This section is unnecessary and confusing, if you're introducing an additional heat source, bump up temp_conv and evaluate it as normal.  If you're concerned about a spike in heat confusing the player, output some log messages letting them know what's happening.