Rename SocketPeerInfo to SocketEndpointInfo.

I'm preparing to be able to ask about the other end of the connection
too, so the first step is to give this data structure a neutral name
that can refer to either. No functional change yet.

proxy/sshproxy.c

@@ -123,7 +123,7 @@ static const char *sshproxy_socket_error(Socket *s)
     return sp->errmsg;
 }
 
-static SocketPeerInfo *sshproxy_peer_info(Socket *s)
+static SocketEndpointInfo *sshproxy_peer_info(Socket *s)
 {
     return NULL;
 }