800K disk images; split out isUniDosDisk from isDosDisk; changed
sizing of image in memory just use the File as that is better than the
dumb logic that was originally in place.
reason, some DOS 3.3 images do not set the VTOC byte 0x03 to be
3 - this is the DOS release number which created the disk. The images
I have just have a zero (0) in this position.