Trac Ticket 26 - Please support setting
                     defaultNamingContext in the rootdse.
    
    https://fedorahosted.org/389/ticket/26
    
    Fix descriptions:
    1) Introducing an attribute defaultNamingContext to rootdse.
    2) To support it, a config parameter nsslapd-defaultnamingcontext
       is added.
       . Suffix created in setup is set to nsslapd-defaultnamingcontext
         in createConfigFile (DSCreate.pm).
       . If the default naming context is deleted from mapping tree,
         the config parameter nsslapd-defaultnamingcontext as well as
         the attribute defaultNamingContext in rootdse are removed.
       . When nsslapd-defaultnamingcontext does not exist, there are
         3 ways to set it.
         a) Next added suffix is automatically set.
         b) Add nsslapd-defaultnamingcontext value to cn=config
            using ldap client.
         c) Shutdown the server and add nsslapd-defaultnamingcontext
            value to cn=config
       . nsslapd-defaultnamingcontext value could be replaced with
         other existing suffix with ldap modify operation.