exit when child has exited, not earlier (mh)

With this little addition, the supervisor won't exit until the child process has really exited.

I need this feature because my app has a graceful shutdown.

Already tested it on my machine and works well. Also added few missing semicolons.
Yeah, two reasons: 1. there are other process managers out there with custom kill signals like `SIGINT2` with their own exit handlers doing other stuff. 2. to avoid deadlocks when the upper process has spawned multiple child processes and these child processes have a graceful shutdown procedure (= having a `setTimeout(cb, process.kill)` in them).