revokefs: Avoid symlink path traversal out of basefd in simple cases

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Use glnx_chaseat to ensure that all the attacker-controlled paths end up
inside the basefd directory.

This requires some new helper functions, which will be used in more
complicated syscalls' implementations in subsequent commits.

[smcv: Separated from a larger commit for better reviewability]
Co-authored-by: Simon McVittie <smcv@collabora.com>
Helps: https://github.com/flatpak/flatpak/security/advisories/GHSA-qrwq-7qwx-q9rp

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