[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_debug.c b/ripngd/ripng_debug.c
index 4ed4fa3..78f13a7 100644
--- a/ripngd/ripng_debug.c
+++ b/ripngd/ripng_debug.c
@@ -204,7 +204,7 @@
 }
 
 /* Debug node. */
-struct cmd_node debug_node =
+static struct cmd_node debug_node =
 {
   DEBUG_NODE,
   "",				/* Debug node has no interface. */