os: os.Root: use O_RESOLVE_BENEATH on FreeBSD

[neild]

FreeBSD's openat supports an O_RESOLVE_BENEATH flag which limits path resolution to occur within the starting directory. We should use it when available.

I'm not sure when O_RESOLVE_BENEATH was added; we may need to support a fallback on older FreeBSDs.

[gabyhelp]

Related Issues

• os: os.Root: use openat2 with RESOLVE_BENEATH on Linux #73077

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