- Add struct device::detach_state, which tells the core what state to put
  the device in when it's detached from its driver (on module removal).

  This is a value in the range of 0-4, with 0 being On and meaning 'Do 
  Nothing', 4 being Off, meaing calling ->shutdown() for the device, and 
  1-3 being low-power states, meaning call ->suspend() for the device. 

- Add per-device sysfs file 'detach_state' to control the value of the 
  field. 

- Add device_device_shutdown() function, and call it from bus.c::
  device_detach_driver().