[Dovecot] check password on behalf of user

[Nikolay Shopik]

I'm running KDC with all users and passwords, but have to manage clients 
who doesn't support GSSAPI like mobile phones. Is it possible to dovecot 
SASL check password in KDC when user sends plain text login? I know this 
is possible to do with saslauthd from Cyrus but prefer to stick with 
dovecot SASL.