sd-bus: use the new information in the client's sockaddr in the creds structure

sd-bus: use the new information in the client's sockaddr in the creds structure

Now that clients might convey comm/description strings via the sockaddr,
let's actually use them on the other side, read the data via
getpeername() parse it, and include it in the "owner" creds (which is
how we call the peer's creds).