INT 15 - SYSTEM - later PS/2s - QUERY A20 GATE SUPPORT

	AX = 2403h
Return: CF clear if successful
	    AH = 00h
	    BX = status of A20 gate support (see #00462)
	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
BUG:	at one point early in processing INT 15/AH=24h, the AMI PCI BIOS
	  version 1.00.05.AX1 compares whether AL==03h instead of AL>=03h,
	  thus causing a random branch on any value of AL greater than 3.
SeeAlso: AX=2402h