ipa-kdb: use predefined filters for a wild-card searches
    
    In case we've got a principal name as '*', we don't need to specify
    the principal itself, use pre-defined filter for a wild-card search.
    
    Previously, we had to escape the '*' as specifying it with an explicit
    matching rule would have violated RFC 4515 section 3. However, since we
    don't really need to specify a different matching rule for a wild-card
    search, we can remove this part completely.
    
    Use this change as an opportunity to simplify the code and reduce
    number of duplicated filter constants -- if extra filter is NULL, we can
    simply pass "" and use _EXTRA filter constants to format the final
    filter.
    
    Fixes: https://pagure.io/freeipa/issue/8624
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Robbie Harwood <rharwood@redhat.com>