Int 2F Fn CB0F  - Communicating Applications Specification - Get/set Auto  [F]

   AX = CB0Fh
   DL = subfunction
       00h get current autoreceive state
       01h set autoreceive state
       DH = number of rings before answer, 00h = never

Return: AX = 0000h autoreceive disabled
   AX = N     number of rings before answer
   AX < 0     error code (see #2720)