I've added a compatibility layer for the INI config so that it can be used in PHP-DI config.

In order to avoid conflicts in config keys (between DI config and INI config), INI config is prefixed with old_config (so that it's easy to see which configs will be eventually replaced)

Entry names are formatted like this: old_config.<section>.<key>, for example: old_config.database.host would be the same as calling Config::getInstance()->database['host'].

FYI you can also get a whole section as array, i.e. old_config.database is the same as Config::getInstance()->database.

That also mean that we can alias INI config entries into DI config, e.g.:

return array(
    'logger.level' => DI\link('old_config.log.log_level'),
);

(in that example logger.level is an alias to the old config key, if you are lost by the syntax read here)

I'm not saying we should do all aliases now, but that will be useful to work on #6622 (configuring the logger in DI config while still using the old INI values for BC).