- move list walking and matching to bus.c (since it's a function of the bus driver)
- do specialized walks of the bus's lists when binding; no more callbacks being passed
  to bus_for_each_*.
- take rwsem when adding and removing both devices and drivers. lists of each are now
  fully protected by that rwsem. It's also taken before we walk each list.
- move calls of device_{de,at}tach() to bus_{add,remove}_device() and calls of 
  driver_{de,at}tach() to bus_{add,remove}_driver().