Add explicit requests for several operational attrs
    
    Operational attributes are not returned in searched requests unless
    explicitly requested according to RFC 4512 section 5.1. Therefore to
    get several standard attributes of root DSE we have to request for
    them. The requested attrs are:
     - altServer
     - namingContexts
     - supportedControl
     - supportedExtension
     - supportedFeatures
     - supportedLDAPVersion
     - supportedSASLMechanisms
    
    Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su>