Signal handling is hard! This commit fixes an issue introduced in bb7779d, see this comment for more information.

Symfony registers by default 4 signals, the most common ones: SIGINT,
SIGTERM, SIGUSR1, and SIGUSR2.

When no signal handler are registered, the default behavior is to stop the
execution of the current script, to replication PHP default behavior.

That's why we had a call to exit(0), before
bb7779d and that's why it must be restored.

However, the concerns raised in the
issue are valid, and we
should provided a solution for this.

Now we have the following features:

• By default, we exit, like PHP does by default on SIGINT, SIGTERM;
• It's possible to tell via the event to not exit, by calling
  $event->abortExit();
• It's possible to tell via the event to exit with a custom code, by calling
  $event->setExitCode($code);
• It's possible, via the SignalableCommandInterface to tell via the command to
  not exit, by returning int|false from handleSignal() method
• Fixed case when there is not event to dispatch at all, but the command register some

I think we have all we need now. I added more tests to ensure we don't break
anything in the future.

Sorry for the massive ping here, but this is bit like an a house of cards to
make it work in any case. I hope I didn't miss anything.

ping @akuzia @lcobucci @GromNaN @olegpro