[FrameworkBundle] Additional helper commands to control PHP's built-in web server

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10827
| License       | MIT
| Doc PR        | symfony/symfony-docs#4005

Basically, both the ``server:status`` and ``server:stop`` wouldn't be really reliable if you had stopped the web server by, for example, killing the process. But honestly I don't know how to platform-independently determine if a process is still running given its PID. Maybe such a way could be a good improvement for the Process component.
It should be good to add a call to `posix_setsid()` here to avoid zombie process.

```php
    if ($posix_setsid() < 0) {
        return 1;
    }
```