Convert system_config_form() to implement FormInterface as a base class.

The current helper function system_config_form() we use to add to the form structure could instead implement the FormInterface, and be used as a SystemConfigForm base class that can easily be extended to provide the same functionality.

I think there are currently about 25 calls to the function in core, and these forms will most probably be converted to implement the FormInterface. When they do, they can just extend this class.

Here is just an initial patch, just posting some quick work.