IMAP: refuse to work when STARTTLS is required but server sends PREAUTH

Oops, we cannot send STARTTLS when the connection is already authenticated.
This is serious enough to warrant an error; an attacker might be going after a
plaintext of a message we're going to APPEND, etc.

Thanks to Arnt Gulbrandsen on the imap-protocol ML for asking what happens when
we're configured to request STARTTLS and a PREAUTH is received, and to Michael M
Slusarz for starting that discussion.

Hope the error message is readable enough.

CVE: CVE-2014-2567