[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/isisd.c b/isisd/isisd.c
index 7c669fc..1e84a1c 100644
--- a/isisd/isisd.c
+++ b/isisd/isisd.c
@@ -2076,7 +2076,7 @@
   return write;
 }
 
-struct cmd_node isis_node = {
+static struct cmd_node isis_node = {
   ISIS_NODE,
   "%s(config-router)# ",
   1