Add X-Ldap-GroupDN for search user in group without memberOf.

[SlavaRejik]

Not all ldap servers have memberOf attribute. If you need to check that the user is in the group, you need to search by filter (memberUid=user) in this groupDN. I add optional X-Ldap-GroupDN parameter for this.

[schittli]

Does nginx inc. really need years to merge this great merge request?

Or does nginx inc. expects that customers merge this request in their own fork??

LDAP Auth is a basic and sometimes very useful feature.