We're logging SUCCESS even when the KDC sends error replies. That's
because we're returning success to process_request() even when we send
errors to clients. The error we want to send to the client, and that we
succeed or fail to send it, are different statuses.
To fix this we'll add a `ret` field to the common service request state
structure, HEIM_SVC_REQUEST_DESC_COMMON_ELEMENTS /
heim_svc_req_desc_common_s.