Fix #568: Added possibility to retrieve operational attributes

As described in http://www.sogo.nu/bugs/view.php?id=568, SOGo lacks the possibility to retrieve operational LDAP attributes (like _memberOf_). This is caused by removing _searchAttributes_ in the belief that "\*" does the same. This is correct, except for operational attributes.

To fix this, I added the optional LDAP source configuration _LDAPLookupFields_. It's default value is _("\*")_ . Therefore it is fully backward compatible.

In my case, I can now have the following _sogo.conf_

      SOGoUserSources = (
        {
          type = ldap;
          
          LDAPLookupFields = ("*", "memberOf");
          
          ModulesConstraints = {
            Mail     = { memberOf = "cn=aclSogoMail,ou=Groups,dc=example,dc=com"; };
            Calendar = { memberOf = "cn=aclSogoCalendar,ou=Groups,dc=example,dc=com"; };
            Contacts = { memberOf = "cn=aclSogoContacts,ou=Groups,dc=example,dc=com"; };
          };
        }
      );

separate question: Can _searchAttributes_ be removed from the Interface?
_LDAPLookupFields_ should be described in the Installation Guide (but I didn't date to touch the *.odt file)