ldap2: use LDAP whoami operation to retrieve bind DN for current connection
For external users which are mapped to some DN in LDAP server, we
wouldn't neccesary be able to find a kerberos data in their LDAP entry.
Instead of searching for Kerberos principal use actual DN we are bound
to because for get_effective_rights LDAP control we only need the DN
itself.
Fixes https://pagure.io/freeipa/issue/6797
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>
Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>