Change C functions to C++ functions in DSFMLC

[Jebbs]

While the C functions work, I think switching to C++ functions would be better.

The main reason is that they have the benefit of leveraging better mangling than C uses, and this would provide a safer binding layer when linking. That is, if the function signatures do not match there will be a linker error.

There will need to be some creativity happening as a few types will no longer match on both ends, but I think it would be worth it in the long run.