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INT 21 - DOS 2+ internal - RETURN CritSectFlag (InDOS) POINTER
        AH = 34h
Return: ES:BX -> 1-byte DOS "Critical Section Flag", also known as InDOS flag
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)
SeeAlso: AX=5D06h, AX=5D0Bh