Extensions homogenization, cleaned up.

Rebased and cleaned up version of extension homogenisation. I will give some comments that should be fixed before merge now.
Yes it's complicated. The `default_config` value contains various global settings to the behavior of the server (especially if he has to follow symbolic links or not). In the engine of extensions, this value is given to extensions that can modify using [Ext_continue_with by example](https://github.com/ocsigen/ocsigenserver/blob/master/src/extensions/userconf.ml#L79) (to create dynamic sub-site).