completion: try to complete copy or command

[martinetd]

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When using clush -c we should try to complete local files.

When not using -c, if a taget has already been set we can also try to complete a command. We could do like ssh and try to complete it on any node in the nodeset, but a local completion is probably good enough.

And still complete possible options but only do so if a dash was present, this is similar to what e.g. ls completion does

Fixes #584