Support for different releases of the National disc (noplus, plusplus and star)

[simoninns]

It seems that there are 3 different copies of the National disc available - the type can be determined from a symbol on the LaserDisc's label:

No symbol - "noplus" version
"++" - "plusplus" version
"*" - star version

Although it's not currently certain, it looks likely that there are differences in the VFS data between the different versions. This issue is a place holder to remind about the required support once the EFM images from the various discs have been analysed.