Fine-tuning penalties for surface=mud and surface=sand

Reading pbfgraphparser.cc, autocost.cc, bicyclecost.cc, pedestriancost.cc, truckcost.cc, etc. I think OSM's surface=mud should be avoided more strongly by pedestrians and motor vehicles. It should be treated similar to surface=sand, but mapping it to kGravel would probably conflict with the routing logic (minimum allowed way type) for special types of bicycles, so maybe a new internal type is needed to handle both well. Like tracktype=grade5, these surfaces are made of soft materials and pose a greater risk of getting stuck or dirty, especially after heavy rain.