AD: Add additional service to support Global Catalog lookups
    
    When fixed host names of AD servers are configured in the config file,
    we can't know (unlike when service discovery is at play) if the servers
    are Global Catalogs or not. This patch adds a private data to servers
    read from the config file that denote whether the server can be tried
    for contacting the Global Catalog port or just LDAP. The GC or LDAP URIs
    are generated based on contents of this private data structure.
    
    Because SSSD sticks to a working server, we don't have to disable or
    remove the faulty GC servers from the list.