Return in-DOS flag (INT 21h, 34h)

        Entry:
          AH = 34h
        Return:
          ES:BX points to IN-DOS flag

        Notes:
          When both the IN-DOS flag and the Critical Error Flags is zero,
          DOS is supposed to be safe to interrupt.

        Notes:
        When the critical section flag is nonzero, code within DOS is being
        executed.  It is safe to enter DOS when both the critical section
        flag and the critical error flag are zero.
        The critical error flag is the byte after the critical section flag
        in DOS 2.x, and the byte BEFORE the critical section flag in DOS 3.x
        (except COMPAQ DOS 3.0, where the critical error flag is located 1AAh
         bytes BEFORE the critical section flag)
        For DOS 3.1+, an undocumented call exists to get the address of the
        critical error flag (see AX=5D06h)