Add PidFd::{kill, wait, try_wait}

[the8472]

#117957 changed Child kill/wait/try_wait to use its pidfd instead of the pid, when one is available.
This PR extracts those implementations and makes them available on PidFd directly.

The PidFd implementations differ significantly from the corresponding Child methods:

• the methods can be called after the child has been reaped, which will result in an error but will be safe. This state is not observable in Child unless something stole the zombie child
• the ExitStatus is not kept, meaning that only the first time a wait succeeds it will be returned
• wait does not close stdin
• wait only requires &self instead of &mut self since there is no state to maintain and subsequent calls are safe

Tracking issue: #82971