[Command] Added an helper to lock a command

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9357 , #3586
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/3956/files

Code sample:

```php
    /**
     * {@inheritdoc}
     */
    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $lockHelper = new LockHelper('/tmp/acme/hello.lock');
        if (!$lockHelper->lock()) {
            $output->writeln('The command is already running in another process.');

            return 0;
        }

        $output->writeln(sprintf('Hello <comment>%s</comment>!', $input->getArgument('who')));

        for (;;) {
        }

        $output->writeln(sprintf('bye <comment>%s</comment>!', $input->getArgument('who')));

        $lockHelper->unlock();
    }
```

![process-lock](https://f.cloud.github.com/assets/408368/2443205/4f0bf3e8-ae30-11e3-9bd4-78e09e2973ad.png)

ah right... I am used to real files, but since you are using `flock` this is not a problem