[Virtio-fs] [virtiofsd] MR opened: Add (and prefer) “prefer” alias to --inode-file-handles

[virtiofs-bot at sinrega.org]

As written in #20, the current name of the `fallback` mode for `--inode-file-handles` can be misleading.  This series first adds a `prefer` alias, then deprecates the `fallback` value, and finally changes the internal `InodeFileHandlesMode` enum to call it `Prefer` instead of `Fallback`, too.

**Note**: Patch 3 will conflict with !96.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/100