[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/ripngd/ripng_zebra.c b/ripngd/ripng_zebra.c
index 62185c0..4c44455 100644
--- a/ripngd/ripng_zebra.c
+++ b/ripngd/ripng_zebra.c
@@ -538,7 +538,7 @@
 }
 
 /* Zebra node structure. */
-struct cmd_node zebra_node =
+static struct cmd_node zebra_node =
 {
   ZEBRA_NODE,
   "%s(config-router)# ",