When using IMAP as auth module every single request causes a warning to
be logged just because we're not encrypting traffic sent to another
process on the *same* machine.
This change recognizes that while some people might consider this
undesirable, others might have made this a conscious choice and *don't*
wish to be spammed for it. As such now only a single warning is logged
(the first time in the server's lifetime that a user logs in).
Signed-off-by: Giel van Schijndel <me@mortis.eu>
