fix bugs in `find_src_files()`.

fix bugs in `find_src_files()`.

Use `_` to avoid superfluous stat.

Check for `defined $file`, instead of just `$file`.

Add spaces after commas.

Change return values of `verify_src_file()` to not return the tainted filename.
Rename `$f` to `$file_untainted in `verify_src_file()`.
$f changes to `$file` in `find_src_files()`.

This attempts to fix commit f3abeac919c4736429bd3362af6edf51ede8e7fe.
For discussion see
<http://ikiwiki.info/todo/auto-create_tag_pages_according_to_a_template/>