The controller now passes the requested attributes to the backend in
ObjectAttributes, so posix and azure only do the extra HeadObject for the
parts count when ObjectParts was asked for. This also lets the s3 backend
reach the upstream service, which rejected the request before because the
field was missing.
The s3 backend now copies TotalPartsCount into PartsCount, leaves out
ObjectParts when the upstream response has none, and sets the part number
markers, which were dropped by inverted strconv.Atoi error checks.