driver model: power management/shutdown cleanup.

- take device_sem around all global list walks.
- don't modify refcount, as get_device()/put_device() also take device_sem
- But, make sure we check device_present() to make sure device is still around.

(Note device removal will block until device_sem is dropped, so list will remain
intact.)

- Separate out device_shutdown() walk from device_suspend() walk. Even though the
  code is nearly identical, it's a lot clearer as to what is going on when they 
  are autonomous. It was my bad for originally putting that FIXME in there, encouraging
  the consolidation. 

- Add debugging hooks for my convenience. :)

- Call ->shutdown() when shutting down device, instead of ->remove(). (See ChangeSet 
  1.799 for description and semantics).

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