cmd: use flag.Value where enumerated options exist or multiple value can be handed in

[kortschak]

See https://bitbucket.org/ausocean/av/pull-requests/173/revid-expose-raspivid-brightness#comment-96126866

But also the sender destinations should really be a single option with a set of URL targets.

[Saxon Milton]

command line version no longer exists, so this is no longer relevant

[kortschak]

I'd say this is still relevant to rvcl.

[Saxon Milton]

rvcl has been moved to the exp directory, and will probably be deleted. It doesn’t do what it used to do with flags anyways.