Support ascii chars in files.

Add support for all possible 7-bit ascii characters in filenames for --preload-file, notably ', " and @. After this commit, all 7-bit non-control ascii characters except `/`, `\` and `:` can be used in filenames when packaging the VFS. The character `@` can also be used, but it must be escaped to form `@@` to not confuse with the `src@dst` placement notation we are using.