Modify the default values for optional parameters.

     SET DEFAULT
         [ DIRECTORY_SERVICE dir-service ]
         [ DISPLAY_MODE display-mode ]
         [ NSAP_FORMAT format ]
         [ PHASEIV_PREFIX addr-prefix ]
         [ REVERSE_DIRECTORY rev-dir-name ]
         [ SYNONYM_DIRECTORY syn-dir-name ]

 This provides a means to preset the values for commonly used parameters,
 such as the reverse address mapping directory name to use for a particular
 directory service.

 This is particularly useful when done in a DECNET_REGISTER initialization
 command file (see "HELP USAGE INITIALIZATION_COMMAND_FILE").