[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/isisd/isis_circuit.c b/isisd/isis_circuit.c
index af24988..d2923b5 100644
--- a/isisd/isis_circuit.c
+++ b/isisd/isis_circuit.c
@@ -2028,7 +2028,7 @@
 }
 #endif /* HAVE_IPV6 */
 
-struct cmd_node interface_node = {
+static struct cmd_node interface_node = {
   INTERFACE_NODE,
   "%s(config-if)# ",
   1,