[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/ripd/ripd.c b/ripd/ripd.c
index cb2603d..0e3f877 100644
--- a/ripd/ripd.c
+++ b/ripd/ripd.c
@@ -3680,7 +3680,7 @@
 }
 
 /* RIP node structure. */
-struct cmd_node rip_node =
+static struct cmd_node rip_node =
 {
   RIP_NODE,
   "%s(config-router)# ",