[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/ospf6_top.c b/ospf6d/ospf6_top.c
index d45d132..609df41 100644
--- a/ospf6d/ospf6_top.c
+++ b/ospf6d/ospf6_top.c
@@ -650,7 +650,7 @@
 }
 
 /* OSPF6 node structure. */
-struct cmd_node ospf6_node =
+static struct cmd_node ospf6_node =
 {
   OSPF6_NODE,
   "%s(config-ospf6)# ",