Implement a LOCAL_PEERCRED socket option which returns a

Implement a LOCAL_PEERCRED socket option which returns a
`struct xucred` with the credentials of the connected peer.
Obviously this only works (and makes sense) on SOCK_STREAM
sockets.  This works for both the connect(2) and listen(2)
callers.

There is precise documentation of the semantics in unix(4).

Reviewed by:	dwmalone (eyeballed)