SF_NOMULTIPLAYER skinflag

This is my first ever "true" merge request, so I hope this works properly.

This MR adds the SF_NOMULTIPLAYER skin flag, which prevents access to a specific character in the Multiplayer Player Setup menu. Simple as that. (Totally not for my own reasons and TOTALLY unrelated to a certain fighter)

I already compiled and tested the build on my own system. Here's the executable, and here's the test file (basically Sonic with an added SF_NOMULTIPLAYER flag - the same one you see in the gif).