Parses an SCA subsystem command and invokes command processing if
    the command is syntactically correct.

    Format

      SCA$DO_COMMAND  command_context,

                      command_string

                      [,parameter_routine]

                      [,continuation_routine]

                      [,continuation_prompt]

                      [,user_argument]

                      [,confirm_routine]

                      [,topic_routine]

                      [,display_routine]