kernel - devfs cleanup

kernel - devfs cleanup

* staticize two functions, remove *_fff() from manual pages (now
  static).

* Adjust documentation.

* Rename internal function from devfs_clone_bitmap_resize() to
  devfs_clone_bitmap_extend().  Document the slight weirdness in
  the extent of the passed-in 'newchunks' parameter, but do not change
  the API.

Submitted-by: Aaron LI, with added code comments by Matt