bundle2: introduce `replycaps` part for on-demand reply
The bundle2 processing does not create a bundle2 reply by default anymore. It
is only done if the client requests it with a `replycaps` part. This part is
called `replycaps` as it will eventually contain data about which bundle2
capabilities are supported by the client.
We have to add a flag to the test command to control whether a reply is
generated or not.
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