Unlock Extended Memory Block (Function 0Dh)

        ARGS:   AH = 0Dh
                DX = Extended memory block handle to unlock

        RETS:   AX = 0001h if the block is unlocked, 0000h otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = 81h if a VDISK device is detected
                BL = A2h if the handle is invalid
                BL = AAh if the block is not locked

This function unlocks a locked extended memory block.  Any
32-bitpointers into the block become invalid and should no longer be
used.