Add address towers to a node registration.

     ADD [DECNET] NODE node-name
         TOWERS {tower-set}
         [ DIRECTORY_SERVICE dir-service ]
         [ PHASEIV_PREFIX addr-prefix ]
         [ REVERSE_DIRECTORY rev-dir-name ]

 This is used when a new address tower is available on a node, and the
 addressing information needs to be added into the node registration in a
 directory service.