      fhead (fitsfile) ... [-x N]

      print out the FITS header, adding return chars to the end of each
      line, stopping at the END (not printed).  By default, the primary
      header unit is printed.  If the -x is provided the Nth extension's
      header unit is printed, with 0 being the first extension after the
      PHU.  Multiple files may be provided, in which case the headers are
      printed in sequence, with tags between the headers.