[Process][FR] Add an option to redirect any output to /dev/null

As well as it could be nice to flush the output, discarding it could be nice in term of memory consumption for long process where output is not needed.

In terms of `proc_open` descriptors, it consists in : 

``` php
$nullfile = defined('PHP_WINDOWS_VERSION_BUILD') ? 'NUL' : '/dev/null';
$descriptors = array(
    1 => array("file", $nullfile, "a+"),
    2 => array("file", $nullfile, "a+"),
);
```
