[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/ripngd/ripng_debug.c b/ripngd/ripng_debug.c
index 4ed4fa3..78f13a7 100644
--- a/ripngd/ripng_debug.c
+++ b/ripngd/ripng_debug.c
@@ -204,7 +204,7 @@
 }
 
 /* Debug node. */
-struct cmd_node debug_node =
+static struct cmd_node debug_node =
 {
   DEBUG_NODE,
   "",				/* Debug node has no interface. */
diff --git a/ripngd/ripng_interface.c b/ripngd/ripng_interface.c
index d76e3a1..d70c299 100644
--- a/ripngd/ripng_interface.c
+++ b/ripngd/ripng_interface.c
@@ -1200,7 +1200,7 @@
 }
 
 /* ripngd's interface node. */
-struct cmd_node interface_node =
+static struct cmd_node interface_node =
 {
   INTERFACE_NODE,
   "%s(config-if)# ",
diff --git a/ripngd/ripng_zebra.c b/ripngd/ripng_zebra.c
index 62185c0..4c44455 100644
--- a/ripngd/ripng_zebra.c
+++ b/ripngd/ripng_zebra.c
@@ -538,7 +538,7 @@
 }
 
 /* Zebra node structure. */
-struct cmd_node zebra_node =
+static struct cmd_node zebra_node =
 {
   ZEBRA_NODE,
   "%s(config-router)# ",
diff --git a/ripngd/ripngd.c b/ripngd/ripngd.c
index 7b00e03..9deac03 100644
--- a/ripngd/ripngd.c
+++ b/ripngd/ripngd.c
@@ -2682,7 +2682,7 @@
 }
 
 /* RIPng node structure. */
-struct cmd_node cmd_ripng_node =
+static struct cmd_node cmd_ripng_node =
 {
   RIPNG_NODE,
   "%s(config-router)# ",