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
Reviewed-by: Pavel Březina <pbrezina@redhat.com>