broker: ensure CURVE certificate has a name

Problem: internally generated curve certs are not named,
so overlay_cert_name() can return NULL, but a name is required
when authorizing a cert.

This API inconsistency results in extra code and confusion
when implementing a new boot method.

Use the rank as the name for internally generated certs.

Author: garlick

src/broker/overlay.c

@@ -279,6 +279,11 @@ int overlay_set_topology (struct overlay *ov, struct topology *topo)
 
     ov->size = topology_get_size (topo);
     ov->rank = topology_get_rank (topo);
+    if (!zcert_meta (ov->cert, "name")) {
+        char buf[16];
+        snprintf (buf, sizeof (buf), "%lu", (unsigned long)ov->rank);
+        zcert_set_meta (ov->cert, "name", "%s", buf);
+    }
     ov->child_count = child_count;
     if (ov->child_count > 0) {
         int i;