#3 GSSAPI/kerberos support

[seph]

It would be neat if there was GSSAPI support. For
example, if one is using mod_auth_kerb, then there's
already kerberos tickets. The cyrus tools are capable
of using that for authentication.

I haven't stared at the websieve code enough, but it
oughtn't be very hard. get the username from the
REMOTE_USER variable, and then just invoke the
sieve_get_handle stuff with it, skipping the whole
login screen.