INT 21 - DOS 5+ - GET OR SET UMB LINK STATE
        AH = 58h
        AL = subfunction
            02h get UMB link state
                Return: AL = 00h UMBs not part of DOS memory chain
                           = 01h UMBs in DOS memory chain
            03h set UMB link state
                BX = 0000h remove UMBs from DOS memory chain
                   = 0001h add UMBs to DOS memory chain
Return: CF clear if successful
        CF set on error
            AX = error code (01h) (see AH=59h)
Note:   a program which changes the UMB link state should restore it before
          terminating

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