Fixes #13639.

shred's pre-checks used Path::exists() and Path::is_file(), both of which return false for any metadata access error, including a permission error. As a result, a file whose parent directory lacks search permission was reported as:

shred: locked/file: No such file or directory

even though it exists.

This replaces the two checks with a single fs::metadata() match:

• a genuine ENOENT still yields "No such file or directory";
• an existing non-file still yields "Not a file";
• a metadata error such as a permission error now falls through to the open-for-writing path, which surfaces the real reason instead of masquerading as a missing file.

All the normal cases (missing file, dangling symlink, directory, regular file) keep their existing behaviour. Added a #[cfg(unix)] regression test that shreds a file inside a 0o000 directory and asserts the error mentions "Permission denied" and not "No such file"; it fails on the old Path::exists() code and passes with the fix. This mirrors the fix in #9789 (chmod).