Get address of critical error flag (INT 21h, 5D06h)

        Entry:
          AX = 5D06h
        Return:
          DS:SI = pointer to critical error flag
          CX = ???
          DX = ???

        Note: This call also does a lot of other work in addition to
              returning the pointer.
        Note: Setting CritErr flag allows use of functions 50h/51h
              from INT 28h under DOS 2.x by forcing use of correct
              stack.