repart: move definitions + dry_run + empty fields into Context

repart: move definitions + dry_run + empty fields into Context

This is preparation for making this eventually available via Varlink,
where we'd like to create Context object for each call that we can free
once it is done, but not inherit state from an earlier call.

Also fixes a couple of cases where we accessed arg_node, but where we
should have accessed the Context-specific copy in .node.