Add the sysinit hook.

In a current customer project I have the problem that I need to manipulate the system environment, right before the request processing started. I tried a lot other hooks from `generateFrontendUrl` till `loadLanguageFile` but they all too late.

I need to manipulate the session data, right before request processing started, because it have also have to affect the `index.php` processing. The really last point to do this is the `initconfig.php`

When adding my code to the `initconfig.php` it works fine, but my customer says it have to be installable without custom modifications on the installation settings. So I request the `sysinit` hook. I called it `sysinit` in souvenir to the sysinit system initialization program for Linux. :-)

I also write the documentation for the hook https://github.com/tristanlins/docs/blob/3.0/hooks/hooks/sysinit.md

I'm hopeful it will be added to 3.1 :-)