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.

defs.h

@@ -44,6 +44,7 @@ typedef struct SockAddr SockAddr;
 
 typedef struct Socket Socket;
 typedef struct Plug Plug;
+typedef struct SocketPeerInfo SocketPeerInfo;
 
 typedef struct Backend Backend;
 typedef struct BackendVtable BackendVtable;