rename "PLATFORM_AGENT_SHUTDOWN" -> "PLATFORM_AGENT_SHUTDOWN_CHILDREN" f...

...or the platform shutdown command.  

https://jira.oceanobservatories.org/tasks/browse/OOIION-1303


NOTE: the resulting message is still the current one (just OK). A more qualified message like saying how many children were shutdown, or that no children were running is not implemented yet.