[ingo] gssapi authenticated Sieve servers

[liamr at umich.edu]

I've written a driver for Ingo to allow it to communicate with Sieve servers
that use GSSAPI authentication.  It requires the sivtest / imtest utility that
comes with the Cyrus IMAP distribution, but seems to be working fine in testing
here at UMich.  Ideally, this would support any authentication mechanism
supported by sivtest, but that's not been a high priority for me.

It's a modified version of the normal Sieve driver that uses sivtest to open the
socket and authenticate.  Due to the nature of the authentication mechanism, it
has it's own mechanism to follow referals as it cannot use the one provided in
Net_Sieve.

The drive also requires a couple of line modification to Net_Sieve which tell
Net_Sieve to skip the authentication stage when the authentication type is set
to GSSAPI, These /may/ get added to Net_Sieve release - I've discussed it with
the author but I'm not sure where things stand.  If they do, they'll probably
be in a form that makes more sense to that module's author.

Anyways. It needs some cleaning up.  I'd welcome feedback, help and suggestions.
It's available at:

    http://www.umich.edu/~liamr/horde/ingo-sivtest.tar

and contains the diff needed for Net/Sieve.php, the driver that gets installed
in ingo/lib/Driver, and example configuration stanza for
ingo/config/backends.php

Liam