[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/ripngd.c b/ripngd/ripngd.c
index 7b00e03..9deac03 100644
--- a/ripngd/ripngd.c
+++ b/ripngd/ripngd.c
@@ -2682,7 +2682,7 @@
 }
 
 /* RIPng node structure. */
-struct cmd_node cmd_ripng_node =
+static struct cmd_node cmd_ripng_node =
 {
   RIPNG_NODE,
   "%s(config-router)# ",