INT 15 U - DOS 3.0+ PRINT.COM - SET CRITICAL REGION FLAG                        
	AX = 2001h
	ES:BX -> byte which is to be incremented while in a DOS call
Return: nothing
Desc:	specify a user flag which PRINT should set to let an interested
	  application know it is in a critical region
Note:	also supported by PCIBM PC Network v1.00 RECEIVER.COM
SeeAlso: AX=2000h