[Virtio-fs] [virtiofsd] MR merged: Clean up the command line usage message

[virtiofs-bot at sinrega.org]

In the usage message, we show legacy and deprecated options whose use we discourage without any warning or deprecated mark.

We can adopt a deprecation policy, for instance, where a deprecated option will be removed in the next major revision.
In this MR the legacy options are marked as "deprecated" and a warning message is shown if are used.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/69