[Virtio-fs] [virtiofsd] MR merged: Adds safe versions of `libc::mount()` and `libc::umount2()`

[virtiofs-bot at sinrega.org]

As part of the effort to remove `unsafe` blocks to increase code
clarity, the wrappers for `libc::mount()` and `libc::umount2()`
are added and used inside sandbox.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/127