MINOR: Add TO/FROM_SET flags to struct stream_interface

[WT: it will make sense to remove SN_FRT_ADDR_SET and to use these
  flags everywhere instead ]
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 6324196..39ef26b 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -443,6 +443,7 @@
 		if (getsockname(fd, (struct sockaddr *)&si->addr.from, &addrlen) == -1) {
 			Warning("Cannot get source address for logging.\n");
 		}
+		si->flags |= SI_FL_FROM_SET;
 	}
 
 	fdtab[fd].owner = si;