[Virtio-fs] [virtiofsd-rs] MR opened: Enable killpriv v2 support

[virtiofs-bot at sinrega.org]

Most of this series is just setup, introducing the relevant FUSE flags needed to our FUSE types.

When applied, the pjdfstests suite now passes without any failures since the killpriv v2 semantics are upheld by the server.

Signed-off-by: Connor Kuehl <ckuehl at redhat.com>
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https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/31