[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/ospf6d/ospf6d.c b/ospf6d/ospf6d.c
index 1a615ff..7a766c8 100644
--- a/ospf6d/ospf6d.c
+++ b/ospf6d/ospf6d.c
@@ -92,7 +92,7 @@
   return CMD_SUCCESS;
 }
 
-struct cmd_node debug_node =
+static struct cmd_node debug_node =
 {
   DEBUG_NODE,
   "",