+ *
+ * If `backend_required' is TRUE, should also listen for activity
+ * at the backend (rekeys, clientalives, unexpected closures etc)
+ * and respond as necessary, and if the backend closes it should
+ * treat this as a failure condition. If `backend_required' is
+ * FALSE, a back end is not (intentionally) active at all (e.g.
+ * psftp before an `open' command).