Review validation of INI settings

[Girgias]

We have a lot of INI settings that define their custom error handling (e.g. the OnUpdateSaveHandler function for the session.save_handler INI setting) that could possibly use standard INI functions.

One issue, on which we lack coverage, is if INI settings accept the different INI quantity formats that we support (e.g. using G as a multiplier at the end, or using an explicit binary/octal/hexadecimal prefix).

It may also make sense to add new helper functions to parse INI settings (e.g. one dedicated for file permissions)