[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/rip_interface.c b/ripd/rip_interface.c
index 54d357c..827663a 100644
--- a/ripd/rip_interface.c
+++ b/ripd/rip_interface.c
@@ -2051,7 +2051,7 @@
   return 0;
 }
 
-struct cmd_node interface_node =
+static struct cmd_node interface_node =
 {
   INTERFACE_NODE,
   "%s(config-if)# ",