Change a node's registered address tower or Phase IV synonym information.

     MODIFY [DECNET] NODE node-name
         [ DIRECTORY_SERVICE dir-service ]
         [ SYNONYM synonym-name ]
         [ TOWERS {tower-set} ]
         [ PHASEIV_PREFIX addr-prefix ]
         [ REVERSE_DIRECTORY rev-dir-name ]
         [ SYNONYM_DIRECTORY syn-dir-name ]

 This command is used when the registered information for a node needs
 to be changed.