Make the Teletype frontend fail if no user input is available

Other text-based frontends can fail if they don't have a TTY, but the Teletype frontend always assumes it can work, even if input is hooked up to /dev/null. This means Debconf can never fall back to the Noninteractive frontend. This causes problems within noninteractive environments such as CI or similar, if users don't know (or forget) to set DEBIAN_FRONTEND=noninteractive.

Add a prompt to ensure that user input is available. This way, if input is attached to /dev/null and no terminal is available on /dev/tty, Debconf will by default fall back from Dialog (fails if no TTY) to Readline (fails if no TTY) to Teletype (fails if no user input available) all the way to Noninteractive.

Closes: #1061480