INT 21 - DOS 2+ - GET ADDRESS OF INDOS FLAG
        AH = 34h
Return: ES:BX -> one-byte InDOS flag
Notes:  the value of InDOS is incremented whenever an INT 21 function begins
          and decremented whenever one completes
        during an INT 28 call, it is safe to call some INT 21 functions even
          though InDOS may be 01h instead of zero
        InDOS alone is not sufficient for determining when it is safe to
          enter DOS, as the critical error handling decrements InDOS and
          increments the critical error flag for the duration of the critical
          error.  Thus, it is possible for InDOS to be zero even if DOS is
          busy.
        the critical error flag is the byte immediately following InDOS in
          DOS 2.x, and the byte BEFORE the InDOS flag in DOS 3+ (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)
        this function was undocumented prior to the release of DOS 5.0.
SeeAlso: AX=5D0Bh,INT 28