Add pidfd-based implementation

[Aaron1011]

The current unix implementation of relies on handling SIGCHLD signals. This requires that the corresponding signal handler be properly set up - if another library in the process modifies it, it will be impossible to properly wait on child processes.

On recent versions of Linux, it's possible to use pidfds to avoid this problem. Using the libstd support (rust-lang/rust#81825), we can create a pidfd at the same time that a child is spawned. The pidfd can then be waited on using epoll or select (with an optional timeout).

See tokio-rs/tokio#4016 for additional discussion of the benefits of this approach