This PR was merged into the 2.2 branch.

Discussion
----------

[Process][2.2] Fix #8739

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8739
| License       | MIT

This adds a fix to #8739. Whenever a call is done to to any non-blocking methods (`Process::isRunning`, `Process::isStopped`, `Process::isTerminated`, `Process::getStatus`, `Process::getPid`...), buffers are read, and callback executed.

Such code will now work :

```
$process->start(function ($type, $data) {
    echo $data;
});

while ($process->isRunning()) {
    // some stuff
    // callback is executed
}
```

Commits
-------

fa769a2 [Process] Add more precision to Process::stop timeout
57d4159 [Process] Avoid zombie process in case of unit tests failure
3ef517b [Process] Fix #8739
7716fb2 [Process] Add failing test for #8739
bff6f3c [Process] Fix CS

* 2.2:
  corrected English grammar (s/does not exists/does not exist)
  [Process] Add more precision to Process::stop timeout
  [Process] Avoid zombie process in case of unit tests failure
  [Process] Fix #8739
  [Process] Add failing test for #8739
  [Process] Fix CS
  Fixed documentation grammar for AuthenticationManagerInterface::authenticate()
  [Validator] fixed the wrong isAbstract() check against the class (fixed #8589)
  [TwigBridge] Prevent code extension to display warning
  Use strstr instead of strpos

Conflicts:
	src/Symfony/Component/Finder/Shell/Command.php
	src/Symfony/Component/Process/Process.php

* 2.3:
  [Process] Revert change
  [Process] Fix #8746 : slowness added in unit tests since #8741
  [Process] Fix #8742 : Signal-terminated processes are not successful
  corrected English grammar (s/does not exists/does not exist)
  [Process] Add more precision to Process::stop timeout
  [Process] Avoid zombie process in case of unit tests failure
  [Process] Fix #8739
  [Process] Add failing test for #8739
  [Process] Fix CS
  [TwigBridge] removed superflous ; when rendering form_enctype() (closes #8660)
  Fixed documentation grammar for AuthenticationManagerInterface::authenticate()
  [Validator] fixed the wrong isAbstract() check against the class (fixed #8589)
  [TwigBridge] Prevent code extension to display warning
  Fix internal sub-request creation
  [FrameworkBundle] made code more generic
  [Form] Moved auto_initialize option to the BaseType
  Use strstr instead of strpos
  Make sure ContextErrorException is loaded during compile time errors
  Fix empty process argument escaping on Windows
  Ignore null value in comparison validators

Conflicts:
	src/Symfony/Component/Debug/Tests/ErrorHandlerTest.php
	src/Symfony/Component/HttpKernel/Tests/Fragment/InlineFragmentRendererTest.php
	src/Symfony/Component/Process/Process.php