thunderbird-config should support krb5 auth

[broder]

There are two issues with the thunderbird-config currently in -proposed.

First, we need to figure out what configuration option corresponds to the "Use SASL^{WSecure Authentication" check mark.}

The other one is more bizarre. When a user doesn't have an imap/poNN.mail.mit.edu service ticket, Thunderbird appears to not-quite-deterministically fail to acquire the service ticket, resulting in a mysterious error that the server "doesn't support" secure authentication. If you try again enough times, eventually Thunderbird will acquire the service ticket and connect successfully.

For what it's worth, running kvno imap/poNN.mail.mit.edu before starting Thunderbird eliminates that error. That is obviously not the way to fix the problem, though.