Kerberos authentication

[coudot]

In Self Service Password, we added the possibility to bind via Kerberos:

       if ( isset($ldap_krb5ccname) ) {
            putenv("KRB5CCNAME=".$ldap_krb5ccname);
            $bind = ldap_sasl_bind($ldap, NULL, NULL, 'GSSAPI') or error_log('Failed to GSSAPI bind.');
        }

We need to include it in the common lib (see ltb-project/self-service-password#797)