[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_interface.c b/ripngd/ripng_interface.c
index d76e3a1..d70c299 100644
--- a/ripngd/ripng_interface.c
+++ b/ripngd/ripng_interface.c
@@ -1200,7 +1200,7 @@
 }
 
 /* ripngd's interface node. */
-struct cmd_node interface_node =
+static struct cmd_node interface_node =
 {
   INTERFACE_NODE,
   "%s(config-if)# ",