Fixes to RFC2385/MD5 BGP

* bgpd/bgp_network.c: Fix MD5 listen in IPv4 version of bgp_socket() by
  adding listen socket to listen_sockets list so that MD5 passwords can
  get set.
* lib/sockopt.c: (sockopt_tcp_signature) Fix bogus "% Error while applying
  TCP-Sig to session(s)" / "can't set TCP_MD5SIG option" startup error
  messages by not returning error when there isn't one.
diff --git a/bgpd/bgp_network.c b/bgpd/bgp_network.c
index 6d16c6e..99f7113 100644
--- a/bgpd/bgp_network.c
+++ b/bgpd/bgp_network.c
@@ -517,6 +517,7 @@
       return ret;
     }
 
+  listnode_add (bm->listen_sockets, (void *)sock);
   thread_add_read (bm->master, bgp_accept, bgp, sock);
 
   return sock;