)
Call parameters for external A20 handler are:
	AX = function
	    0000h disable A20
	    0001h enable A20
	    0002h get A20 state
Return: AX = status (functions 0000h and 0001h)
	    0000h failure
	    0001h successful
	AX = A20 state (function 0002h)
	    0000h disabled
	    0001h enabled
Note:	HIMEM.SYS only calls function 0002h if the returned CL indicated that
	  the handler supports the call