INT 21 - DOS 3.2+ - IOCTL - GET LOGICAL DRIVE MAP
        AX = 440Eh
        BL = drive number (00h=default,01h=A:,etc)
Return: CF set on error
            AX = error code (01h,0Fh) (see AH=59h)
        CF clear if successful
            AL = 00h block device has only one logical drive assigned
                 1..26 the last letter used to reference the drive (1=A:,etc)
Note:   DR-DOS 3.41-5.0 DRIVER.SYS does not support drive mapping and thus
          always returns AL=00h
SeeAlso: AX=440Fh,INT 2F/AX=122Bh

.