NetPlay: Check file names when receiving GCI folder

[Techjar]

A sort of follow-up to #9754. Has not yet been tested.

[AdmiralCurtiss]

Admittedly this would require a larger rewrite, but... what is the benefit of syncing individual GCI filenames? Wouldn't it be far easier (and likely more consistent, when you think about things like file order returned when iterating through a directory and the like) to just build a raw memory card in memory on the host, and then sync that?

[Techjar]

That doesn't really sound easier. Not only does it make the save syncing code more complex, it also means we have to worry about extracting the raw memory card back to the host's GCI folder if write saves is enabled.

[Pokechu22]

Since / and \ are escaped, path traversal attacks shouldn't be possible, which is good. However, it might be good to also treat empty file names as invalid (this might already be handled by the !std::all_of check, but it could be made explicit). Windows also has some file name restrictions but those are probably not worth worrying about.