Use O_NOFOLLOW_ANY on macOS

According to [this blog post](https://blog.malwarebytes.com/mac/2021/08/macos-11s-hidden-security-improvements/#O_NOFOLLOW_ANY), macos 11 has a new `O_NOFOLLOW_ANY` flag that causes `open` to fail if any path component is a symlink. Assuming symlinks are relatively rare, cap-std could use this as a fast path -- check that the path contains no explicit `..` and then open with `O_NOFOLLOW_ANY`; if that succeeds, then we're done. If either of those fails, open with the slow path.

Start by tracing cap-std's path-opening entry points and the existing fast and slow paths for symlink handling. Check how explicit `..` components are detected and how platform-specific open flags are defined. Done means macOS 11 can use `O_NOFOLLOW_ANY` when safe, while the existing slow path still handles fallback cases.

Written by the indexing model from the issue text.