vty: fix vty->index for implicit go_parent_node

After this patch, most vty_go_parent() functions are really obsolete, as
originally intended: A vty_go_parent() is only needed if the program
requires an action to run on VTY node exit.

vty_transcript_test.vty shows the fixed behavior.

For details, see preceding patch
"vty: show bug in implicit go_parent_node"
I2472daed7436a1947655b06d34eb217e595bc7f3

Change-Id: Id408c678d18ba19b1c1394c3fb657536153d2094
diff --git a/src/vty/command.c b/src/vty/command.c
index 1e76474..a60b544 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -2438,6 +2438,7 @@
 	llist_del(&parent->entry);
 	vty->node = parent->node;
 	vty->priv = parent->priv;
+	vty->index = parent->index;
 	if (vty->indent)
 		talloc_free(vty->indent);
 	vty->indent = parent->indent;
@@ -2648,6 +2649,7 @@
 		struct vty_parent_node this_node = {
 				.node = vty->node,
 				.priv = vty->priv,
+				.index = vty->index,
 				.indent = vty->indent,
 			};
 		struct vty_parent_node *parent = vty_parent(vty);
@@ -2923,6 +2925,7 @@
 		this_node = (struct vty_parent_node){
 				.node = vty->node,
 				.priv = vty->priv,
+				.index = vty->index,
 				.indent = vty->indent,
 			};