cbq-shell: Echo the password input prompt to STDERR when STDOUT is not a terminal

The request is to have cbq-shell notify the user that it is waiting for a password when then user has redirected output to a file.

We should respect shell redirection (it is after all what the user has specified) so echoing to STDERR is a middle ground alerting users not using the provided output options whilst still permitting redirection to eliminate terminal output (just have to redirect STDERR too).