[Process] Add signal and getPid methods

Bug fix: #5030
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
License of the code: MIT

This PR replaces #5391 ; it adds :

 - sigchild compatibility mode. This means that if you activate it you have access to this exitcode of the process in case your php has been compiled with --enable-sigchild. This can happen when you deal with Oracle databases.

```php
$process->setEnhanceSigchildCompatibility(true);
```

 - `getPid` method to get the actual process identifier of the process

```php
$process->getPid();
```

 - `signal` method to send Posix signal to the process

```php
$process->signal(SIGHUP);
```

 - Add optionnal `$signal` as second argument to `stop`method. If provided, the signal is sent at timeout to the process. Example of use :

```php
$process->stop(5, SIGKILL);
```

Tests have been enhanced and now run both sigchild / non-sigchild mode.

By the way, tests are successful with a PHP compiled with the --enable-sigchild option ;)