[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/bgpd/bgp_debug.c b/bgpd/bgp_debug.c
index 1d5bf6b..fa316c5 100644
--- a/bgpd/bgp_debug.c
+++ b/bgpd/bgp_debug.c
@@ -856,7 +856,7 @@
   return write;
 }
 
-struct cmd_node debug_node =
+static struct cmd_node debug_node =
 {
   DEBUG_NODE,
   "",