pidfd and O_NONBLOCK

[Josh Triplett]

As far as I can tell, O_NONBLOCK has no effect on a pidfd. When calling
waitid on a pidfd for a running process, it always blocks unless you
provide WNOHANG.

I don't think anything depends on that behavior. Would it be possible to
make O_NONBLOCK on a pidfd cause waitid on a running process to return
EWOULDBLOCK?

This would make it easier to use pidfd in some non-blocking event loops.

- Josh Triplett