[libata scsi] MODE SELECT, strengthen mode sense

  - move default mode pages to the front of libata-scsi.c
    so various functions can access them
  - partial annotation of these pages, point out divergence
    from sat-r06
  - replace various mode page magic numbers with defines
  - strengthen MODE SENSE command decoding: handle DBD
    bit in cdb, yield block descriptor (per sat-r06) and
    handle mode sub pages

Signed-off-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>