[HttpKernel] switched Kernel::init() method call to userland

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [yes]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [#6834]
| License       | MIT

See #6834, as @fabpot said, a fix would be to move the code to userland, I suggest to keep it in this ```init()``` method though, and tag it as api, for a few reasons : 
- assuming it's content belongs to the dev front controller is wrong, it only depends on the debug mode.
- it can become the recommended place where one can set its application specific php ini settings (not sure if there is such place currently for a symfony project? I usually do this in my front controllers) and also set up its own debugging tools, xhprof and so.