Investigate migrating naive `libc` code to managed/safe `rustix`

[MarijnS95]

Actually I think we should switch to nix or rustix: no code below is validating the error codes returned by libc, while those APIs do.

On quick inspection I prefer rustix: they return OwnedFd from pipe() rather than nix' returning RawFd.

Originally posted by @MarijnS95 in #116 (comment)