vty: indent to exit, side effect

I implemented https://gerrit.osmocom.org/3880 to use indenting for vty implicit 'exit'. There's one thing I'm sneaking in there though which I'd like to mention before I merge: The patch is storing vty parent node state. This state not only stores previous indenting, but it also stores the parent's 'node' id and 'priv' data (the node's object), and puts these back in place after going to a parent. So far our go_parent_cb()s set these, and AFAIK all of them simply put the parent's exact node and priv pointer back in place. But technically, a go_parent_cb() *could* do some magic like going to a completely different node, or re-creating a new priv pointer. By overwriting from stored state, we will override such behavior. I think we want this to happen from stored state in the future, and drop most of the go_parent_cb(). Does everyone agree? ~N