This commit implements configuration reloading with SIGHUP.
There are rough edges left:
* Rename SignalServer with more meaningful name, say, WorkerProcess.
* We should introduce global configuration object which is not
affected by configuration reloading. It should hold cmdcfgs, argc,
argv, and last worker PID.
* We should close the listener file descriptor when some operation was
failed after that.
Tatsuhiro Tsujikawa