[Virtio-fs] [virtiofsd-rs] MR opened: Add flags and options to be compatible with the C implementation (Part 2)

[virtiofs-bot at sinrega.org]

This is the second part of the MRs implementing command line compatibility with the C implementation (the first part is !37).

This series adds:

- --fd
- --socket-path
- --socket-group
- --rlimit-nofile

In addition to them, it also adds support for parsing options in legacy format (`-o`), allowing virtiofsd-rs to be used as a drop-in replacement for the C implementation (tested with libvirt).
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https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/39