pi/netsender.go: add ReponseShutdown functionality to netsender

[Saxon Milton]

This would allow us to make a shutdown request from vidgrind. The syncreboot utility will be used with the “-s” flag to invoke a shutdown. This will be much better then just cutting power to devices.

[Saxon Milton]

Merged in response-shutdown (pull request #97)

pi/netsender/netsender.go: add functionality to deal with ResponseShutdown rc code

resolves issue #55

Approved-by: Trek Hopton

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