Some SSH proxies and jump-hosts authenticate with the "none" method,
requiring no password or key. dbx only tried publickey/password/agent,
so connecting through such a host failed with "SSH public key
authentication failed" — and the frontend forced users to enter a
credential just to pass validation.
- Backend: probe authenticate_none first in connect_and_authenticate;
on rejection fall back to the configured credential method, matching
standard openssh behavior.
- Frontend: drop the "password/key/agent required" check in
validateTransportLayers so passwordless hops can be saved and tested.
Refs #1628
jischeng
849caf4b39