INT 21,34 - Get Address to DOS Critical Flag

INDOS	(Undocumented DOS 2.0+)

	AH = 34h

	on return:
	ES:BX = address of a byte indicating whether a DOS call is
		in progress.  No DOS calls should be made if set.


	- AKA the INDOS flag, this critical section flag may be checked from
	  within an interrupt handler before requesting a DOS service. It is
	  a semaphore that is non-zero when DOS is busy, and zero otherwise.
	- though this flag indicates whether a DOS interrupt is active, it
	  should not be used alone to determine DOS is safe for re-entry;
	  Here's the standard rule for safe DOS entry: if INT 28 is active
	  or this flag and the critical error flag are clear then it is safe
	  to call DOS
	- this interrupt should be used only during TSR initialization;
	  the returned pointer should be used thereafter
	- this flag is cleared after a critical error (INT 24)
	- ES:BX-1 points to the critical error flag for DOS 3.x+
	  ES:BX+1 points to the critical error flag for DOS 2.x
	  ES:BX-1AA points to the critical error flag for COMPAQ DOS 3.0
	  INT 21,5D in DOS 3.x+ can be used to locate the critical
	  error flag