INT 2F U - DOS 3.0+ internal - GET FILE'S DRIVE                                 
	AX = 121Ah
	DS:SI -> filename
Return: AL = drive (0 = default, 1 = A:, etc, FFh = invalid)
	DS:SI -> filename without leading X: (if present)
SeeAlso: INT 21/AH=19h,INT 21/AH=60h