Execute a single DCL command in a subprocess, or create an interactive
 subprocess in which you can enter multiple DCL commands.

     SPAWN [dcl-command]

 If you specify the optional DCL command, it is executed and control
 returns to DECNET_LOC_REGISTER when the execution is complete.

 If you do not specify the optional DCL command, you are placed in a
 subprocess and presented with the DCL prompt.  When you log out of the
 subprocess, control returns to DECNET_LOC_REGISTER.