Disable mbstring.func_overload

[WanWizard]

The Fuel core sort of relies on this feature, which can cause all sorts of misery, and is therefore deprecated in PHP 7.2.

It means using an ini_set() to explicitly disable mbstring.func_overload if mbstring is loaded, scan the entire codebase for functions affected, and per hit decide whether or not it should be a multibyte function.