INT 21 - DOS 1+ - GET STDIN STATUS
        AH = 0Bh
Return: AL = 00h if no character available
           = FFh if character is available
Notes:   Break checked, and INT 23 called if pressed
        standard input is always the keyboard under DOS 1.x, but may be
          redirected under DOS 2+
        if the interim console flag is set (see AX=6301h), this function
          returns AL=FFh if a partially-formed double-byte character is
          available
SeeAlso: AX=4406h