sdap_save_user: try to determine domain by SID
    
    GC contains objects from both parent domain and subdomain.
    
    Lets say we have user with UID 5000 that belongs to a subdomain and
    overlapping search bases dc=ad,dc=pb and dc=sub,dc=ad,dc=pb. Now
    we call 'getent passwd 5000' and this request goes through data
    provider, searching in parent domain first. Even though this
    user does not belong to this domain it is found and stored as
    ad.pb user.
    
    With this patch we look at user's SID and put it into correct domain.