`waitid(P_ALL, ...)` exclusion seems slightly redundant

It seems to me that one can get the same effect by:

- Always using `WNOWAIT` first.
- Remove the PID from systemd's own data structures.  Keep the (valid) pidfd.  If any data needs to be obtained from the zombie process, get it.
- Finally, wait without `WNOWAIT`.

This is admittedly more complex, but it should do the job.