Zipc.FpathFile paths and modes.
ensure_directoryness p makes sure p ends with a '/'. Returns "./" on the empty string.
sanitize_path p is a sanitized for path p. This is either a relative path rooted in the current directory or the empty string. Directoryness (trailing '/') is suppressed and the function is not injective.
No effort is made to interpret relative segments correctly; a zip file with such paths is likely malice or clumsiness. More precisely this splits p on '/' and '\\', removes any ".", ".." or "" segment (in particular this removes absolute paths) and concatenates the resulting list with "/".