Fall back to cn if gecos is not available
    
    We were not fully compliant with section 5.3 of RFC 2307 which
    states:
    
       An account's GECOS field is preferably determined by a value of the
       gecos attribute. If no gecos attribute exists, the value of the cn
       attribute MUST be used. (The existence of the gecos attribute allows
       information embedded in the GECOS field, such as a user's telephone
       number, to be returned to the client without overloading the cn
       attribute. It also accommodates directories where the common name
       does not contain the user's full name.)