FS_U: Implemented the SaveData archive

Implemented the SaveData archive, allows some games to boot further.

Further explanation in the commit messages.
How about: "When a SaveData archive is created for the first time, it is not yet formatted and the save file/directory structure expected by the game has not yet been initialized. Returning the NotFormatted error code will signal the game to provision the SaveData archive with the files and folders that it expects."