Improve sk_peer_info.

Previously, it returned a human-readable string suitable for log
files, which tried to say something useful about the remote end of a
socket. Now it returns a whole SocketPeerInfo structure, of which that
human-friendly log string is just one field, but also some of the same
information - remote IP address and port, in particular - is provided
in machine-readable form where it's available.

errsock.c

@@ -39,7 +39,7 @@ static const char *sk_error_socket_error(Socket *s)
     return es->error;
 }
 
-static char *sk_error_peer_info(Socket *s)
+static SocketPeerInfo *sk_error_peer_info(Socket *s)
 {
     return NULL;
 }