[$] Completing the pidfd API

Over the last few kernel releases, the kernel has gained the concept of a"pidfd" - a file descriptor that represents a process. What started as away of sending signals to processes without race conditions has evolvedinto a more complete process-management interface. Now one of the lastpieces is being put into place: the ability to wait for processes usingpidfds. But, naturally, that API has had to go through some revisionsfirst.