Previously, after sending SIGUSR2 to the original master process, and
the new master process gets ready, user has to send SIGQUIT to the
original master process to shut it down gracefully. With this commit,
the new master process sends SIGQUIT to the original master process
when it is ready to serve requests, eliminating for user to send
SIGQUIT manually.
This works nicely with systemd, because now you can replace nghttpx
binary with new one by "systemctl kill -s USR2 --kill-who=main
nghttpx".
56c455bca4