#363 Minimal snapshot support for Currah uSource

[Fredrick Meunier]

The attached (untested but compiles) patch adds minimal snapshot support for uSource to libspectrum and uses it in Fuse.

No current snapshot formats support saving uSource information but this causes Fuse to correctly flag the information loss on saving the snapshot.

It should be simple to define a SZX block for uSource based on this structure and add support for it but I don't think that the release needs to be blocked for that support to be added.

I'd appreciate any comments or testing and I hope to add this patch by the weekend.

[Sergio Baldoví]

Just some small fixes: don't crash and be C89 friendly.

[Fredrick Meunier]

Thanks Sergio, I fixed another of my bugs and committed in [r5447] and [r5448].