* Remove all partition awareness and device number interpretation 
  from the fs code.  

  [This is DONE but in a different manner than described below (which
   is retained for future consideration).  The fs_netbsd_mount
   interface was changed to take a blkio, flags, and an out_fs.  The
   blkio is usually a blkio_subset and the pointer value is used for
   the dev_t.  We redefined the major() macro to return 0 and then put
   "wd" as entry zero of the bdev_sw.  Then all we needed to define
   was wdopen, wdstrategy, etc, which call the blkio functions.]

  (1) Complete com/blkio_subset.c to support representing
      partitions with oskit_blkio_t's, as opposed to only
      representing entire block devices.

  (2) Change the fs_netbsd_mount interface to be of the 
      form fs_netbsd_mount(type,dev,iop,flags,out_dir),
      where dev is an _opaque_ device number to use for
      any stat operations on files within the filesystem,
      and iop is a oskit_blkio_t* which represents the
      desired partition.

  (3) Update spec_vnops.c to use blkio operations rather
      than invoking bdevsw operations.