INT 15 - SYSTEM - later PS/2s - ENABLE A20 GATE

	AX = 2401h
Return: CF clear if successful
	    AH = 00h
	CF set on error
	    AH = status
		01h keyboard controller is in secure mode
		86h function not supported
Notes:	also supported by AMI PCI BIOS and Qualitas 386MAX v6.01+
	IBM classifies this function as optional
SeeAlso: AX=2400h,AX=2402h,PORT 0092h