[cleanup] Make command nodes static

The cmd_nodes used to configure vty, can mostly be static so
(basic data hiding 101).
diff --git a/lib/plist.c b/lib/plist.c
index 4b498c4..0f802a8 100644
--- a/lib/plist.c
+++ b/lib/plist.c
@@ -2623,7 +2623,7 @@
 
 
 /* Prefix-list node. */
-struct cmd_node prefix_node =
+static struct cmd_node prefix_node =
 {
   PREFIX_NODE,
   "",				/* Prefix list has no interface. */
@@ -2732,7 +2732,7 @@
 
 #ifdef HAVE_IPV6
 /* Prefix-list node. */
-struct cmd_node prefix_ipv6_node =
+static struct cmd_node prefix_ipv6_node =
 {
   PREFIX_IPV6_NODE,
   "",				/* Prefix list has no interface. */