[Request]: Add `volume rm --all`, remove unreferenced volumes with `volume prune`.

[jglogan]

Feature or enhancement request details

These are similar enough to implement in one go:

• volume rm --all behaves like network rm --all, it attempts to remove all volumes, and emits error output if any failures occurred, indicating the volumes that weren't removed.
• volume prune removes all volumes that have no container references.

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