Release Upper Memory Block (Function 11h)

        ARGS:   AH = 11h
                DX = Segment number of the upper memory block

        RETS:   AX = 0001h if the block was released, 0000h otherwise

        ERRS:   BL = 80h if the function is not implemented
                BL = B2h if the UMB segment number is invalid

This function frees a previously allocated upper memory block.  When an
UMB has been released, any code or data stored in it becomes invalid and
should not be accessed.