[Process] Fixes issue #11421
[Process] Add test to verify fix for issue #11421
Bring code into standard
bug #11425 Fix issue described in #11421 (Ben, ben-rosio)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix issue described in #11421

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11421
| License       | MIT
| Doc PR        | NA

This pull request fixes the issue described in #11421.  It also adds a test for the issue.  The issue is present in 2.0 forward, but I decided to fix it on the 2.3 branch so that I could also write a test for it (2.0 had no tests for the Process component, and 2.1 and 2.2 didn't have tests for the `ExecutableFinder` class).

Commits
-------

4cf50e8 Bring code into standard
9f4313c [Process] Add test to verify fix for issue #11421
02eb765 [Process] Fixes issue #11421
[Process] fix signal handling in wait()

wait() throws an exception when the process was terminated by a signal.
This should not happen when the termination was requested by calling
either the stop() or the signal() method (for example, inside a callback
which is passed to wait()).
[Process] Fix unit tests in sigchild environment
Merge branch '2.3' into 2.4

* 2.3:
  Update validators.eu.xlf
  fixed CS
  remove unused imports
  Unify null comparisons
  [EventDispatcher] don't count empty listeners
  [Process] Fix unit tests in sigchild environment
  [Process] fix signal handling in wait()
  [BrowserKit] refactor code and fix unquoted regex
  Fixed server HTTP_HOST port uri conversion
  Bring code into standard
  [Process] Add test to verify fix for issue #11421
  [Process] Fixes issue #11421
  [DependencyInjection] Pass a Scope instance instead of a scope name.

Conflicts:
	src/Symfony/Component/EventDispatcher/Tests/EventDispatcherTest.php