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INT 2F - Multiplex - DOS 3+ internal - DECREMENT SFT REFERENCE COUNT
        AX = 1208h
        ES:DI -> SFT
Return: AX = new value of reference count
Note:   if the reference count was 1, it is set to FFFFh (since 0 indicates
          that the SFT is not in use).  It is the caller's responsibility to
          set the reference count to zero after cleaning up.