use pdfork instead of pid

[mheily]

bapt@ suggests using pdfork to avoid the race if a PID is recycled while jobd still has references to it. This might happen once we start caring about child processes of things that we kill, waiting for them to die, and then trying to kill the parent afterword. If the parent process is gone, but the PID is recycled and used by a new process, then jobd might do the wrong thing.

Linux has something similar, but it may be harder to poll.