Add two new options, pin_host_key and host_keys, that
together provide a TOFU host-key validation mode for users who don't
maintain a known_hosts file. When --sftp-pin-host-key is used, rclone
records the server's host key into host_keys on the first successful
connection and verifies it on every subsequent connection.
host_keys is always validated when non-empty, so it can also be used
by hand to pin a known fingerprint without enabling TOFU writing.
known_hosts_file takes precedence if both are set. SSH host
certificates are rejected with a clear message pointing at
known_hosts_file. On-the-fly remotes log a warning since the captured
key cannot be persisted.
Nick Craig-Wood
b2aa82061f