realpath: Report correct path on failure

[des]

Summary

If lstat() fails with EACCES or ENOTDIR, the path we need to return in
the caller-provided buffer is that of the parent directory (which is
either unreadable or not a directory; the latter can only happen in the
case of a race) rather than that of the child we attempted to stat.

Sponsored by: Klara, Inc.

[des]

fix trailing slash