INT 2A,3 - Check if Direct I/O Allowed

	AH = 03
	DS:SI = pointer to ASCIIZ disk device name or pathname (including ":")
	on return:
	CF = set if disk access is NOT allowed
	   = clear if disk access is allowed

	- do not use INT 13, INT 25 or INT 26 if this function returns
	  with the CF set
	- see  INT 5F,2