Thanks for the CC.

It would be nice to get `pidfd_send_signal` as well, while we're at it. But that could be a separate PR.

AFAICT the other bits of the pidfd API right now are some extra flags to clone, and an extra flag to waitid. The waitid flag is nice-to-have but not really urgent, since it's easy enough to use a flag even if the stdlib doesn't explicitly expose it. The clone flags are super interesting, but before we can use them, we need to have some way to access clone itself, and that's a big complicated project, so we definitely shouldn't worry about it here.