sftp: remember entered password in AskPass mode
sftp: remember entered password in AskPass mode

As reported in

  https://github.com/rclone/rclone/issues/4660#issuecomment-705502792

After switching to a password callback function, if the ssh connection
aborts and needs to be reconnected then the user is-reprompted for their
password.  Instead we now remember the password they entered and just give
that back.  We do lose the ability for them to correct mistakes, but that's
the situation from before switching to callbacks.  We keep the benefits
of not asking for passwords until the SSH connection succeeds (right
known_hosts entry, for example).

This required a small refactor of how `f := &Fs{}` was built, so we can
store the saved password in the Fs object

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone