Reduce requirement of Cannon Fodder 2 files

[segrax]

From Issue #32 from andrew-mcmahon

"I couldn't see any terminal output listing missing files when I ran the game so I was wondering why Cannon Fodder 2 wasn't showing up (I only copied CF_ENG.DAT to Dos2_CD)"

We should reduce the requirement from all files down to just CF_ENF.DAT, as it can use the Amiga music instead of the PC files, making all the PC files not necessarily required.

[segrax]

This may require an extension of the detection system, as currently its all-or-none, so perhaps we need a toggle flag added (definite vs optional)