[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_interface.c b/ospf6d/ospf6_interface.c
index 4215208..5a79862 100644
--- a/ospf6d/ospf6_interface.c
+++ b/ospf6d/ospf6_interface.c
@@ -1509,7 +1509,7 @@
   return 0;
 }
 
-struct cmd_node interface_node =
+static struct cmd_node interface_node =
 {
   INTERFACE_NODE,
   "%s(config-if)# ",