[Virtio-fs] [virtiofsd] MR opened: 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, as in the case of "socket".

Although the plan is to support the old options forever,  any new feature must be exposed only using the new cli. Also, we do not have a usage message for most of the legacy options (`-o`) and also there is a mismatch with some new options (e.g. sandbox).

Regarding "socket", we've already replaced it by "socket-path" showing a warning if the "socket" is used.

Here we go one step further by hiding those option from the usage message.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/69