Adds a flag, --progress-terminal-title, that when used with --progress,
will print the string `ETA: %s` to the terminal title.
This also adds WriteTerminalTitle to lib/terminal
Based on Issue 4087
https://github.com/rclone/rclone/issues/4087
Current behaviour is insecure. If the user specifies this value then we
switch to validating the server hostkey and so can detect server changes
or MITM-type attacks.