If you look at the sequence of values returned
by waitpid, it simply tells us about the child of
clone before it tells us that the parent called clone.
There's nothing we can do but assume unexpected
tids are newly cloned children.
Tested with 6prof on godoc.
Fixes#251.
R=r
CC=golang-dev
https://golang.org/cl/2167045
Anthony Martin
Nigel Tao
Russ Cox
Austin Clements
Rob Pike