Prevent possible IMAP MITM via PREAUTH response.
This is similar to CVE-2014-2567 and CVE-2020-12398. STARTTLS is not allowed in the Authenticated state, so previously Mutt would implicitly mark the connection as authenticated and skip any encryption checking/enabling. No credentials are exposed, but it does allow messages to be sent to an attacker, via postpone or fcc'ing for instance. Reuse the $ssl_starttls quadoption "in reverse" to prompt to abort the connection if it is unencrypted. Thanks very much to Damian Poddebniak and Fabian Ising from the Münster University of Applied Sciences for reporting this issue, and their help in testing the fix.