sysdb: include email in UPN searches
    
    Email addresses and Kerberos user principals names (UPNs) do not only
    look similar they also can be used to identify a user uniquely.
    
    In future this approach should be replace by a more generic one where
    the attributes which can uniquely identifies a user can be configured to
    support even a wider range of login names.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>