ldap: add new option ldap_sasl_maxssf
    
    There is already the ldap_sasl_minssf option. To be able to control the
    maximal security strength factor (ssf) e.g. when using SASL together
    with TLS the option ldap_sasl_maxssf is added as well.
    
    Related to https://pagure.io/SSSD/sssd/issue/4131
    
    (cherry picked from commit 3b95c39819591428b1e535e5dc874268cf5630c5)
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>