INT 16 - Doorway v2.x+ - INSTALLATION CHECK / REDIRECTION CONTROLa              
	AH = 67h
	AL = Doorway mode
	    00h turn off DOS redirection
	    01h turn on DOS redirection
Return: AH = 80h if Doorway is actively redirecting output
	    AL = 80h if Doorway was already in requested mode
Program: Doorway is a program by Marshall Dudley which allows most DOS programs
	  to run as a BBS door without modification
Note:	to perform an installation check without changing the redirection
	  state, if is necessary to restore the original mode if AL was not
	  80h on return
SeeAlso: INT 10/AH=BBh"Doorway"