BGP: Better dispatch of incoming connections

Since v2 we have multiple listening BGP sockets, and each BGP protocol
has associated one of them. Use listening socket that accepted the
incoming connection as a key in the dispatch process so only BGP
protocols assocaited with that listening socket can be selected.
This is necesary for proper dispatch when VRFs are used.

sysdep/unix/io.c

@@ -1077,6 +1077,7 @@ sk_passive_connected(sock *s, int type)
 
   sock *t = sk_new(s->pool);
   t->type = type;
+  t->data = s->data;
   t->af = s->af;
   t->fd = fd;
   t->ttl = s->ttl;