SDAP: refactor sdap_access_filter_send
    
    As preparation for ticket #2364 separate code for parsing user basedn
    to a new function sdap_get_basedn_user_entry().
    
    We actually do not need to call strdup on basedn, instead we can just point to address in user_entry as it's allocated on parent memory context.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>