rclone: Path traversal in serve s3 allows reading and overwriting root-level files

### Summary rclone serve s3 allows a client to read and write files at the root of the remote which would normally be inaccessible by using dot-dot path segments in the object key. It does not allow reading files outside of the root. A request such as GET /bucket/../root-secret.txt is handled as an object request for bucket "bucket", but rclone normalizes the backend path and reads root-secret.txt from the serve root. The same issue also allows overwriting root-level files with PUT. ### Details The affected component is rclone serve s3. Relevant source files: cmd/serve/s3/backend.go cmd/serve/s3/multipart.go cmd/serve/s3/list.go In cmd/serve/s3/backend.go, the S3 backend builds backend paths by joining the bucket name and object key with path.Join: fp := path.Join(bucketName, objectName) This pattern is used in object operations such as HeadObject, GetObject, PutObject, DeleteObject, and CopyObject. S3 object keys are opaque names and can legally contain dot-dot segments. However, path.Join treats the object key as a filesystem-style path and normalizes ../ segments. As a result, an object key such as ../root-secret.txt is resolved outside the selected bucket directory. For example, when rclone serve s3 is serving a root directory that contains: root/ bucket/ root-secret.txt a raw S3 HTTP request to: GET /bucket/../root-secret.txt is parsed as a request for bucket "bucket" and object "../root-secret.txt". The backend then calculates: path.Join("bucket", "../root-secret.txt")