CommonFuncs: Set the locale conventions in uselocale().

newlocale() no longer sets the locale categories, so we have to set the locale conventions manually. This fixes LC_NUMERIC_MASK and LC_MONETARY_MASK, but all other masks are still broken.

Though I think it would be a cleaner solution to stop trying to write a POSIX compatibility layer and just provide a thread-safe SetLocale implementation, so that we don't have to use the badly supported locale_t structs on Windows.