handle --relative really well:
- We must set "is_dot_dir" when we strip off a trailing "/" or "/.".
- If a trailing "/" or "/." caused us to treat a symlink as a dir,
  we now pass our stat() info down to make_file() so that it will
  always treat the cleaned-up name as a dir too.
- We must not strip a leading "/".
- Improved the check for ".." elements in the implied dirs.