Windows compatibility: proper path canonicalization for all os versions and cases

[reimic]

As part of the windows compatibility effort a question arose regarding proper canonicalization of paths. To quote @adamziel in a discussion on Symfony's Path method canonicalize:

I'm not sure this is the right approach as it will turn C:\Program Files\Stuff into C:/Program Files/Stuff. It might be fine but I'm not sure if it will universally work with all Windows versions and paths, e.g. what about network shared directories such as \\NetDirectory\SubPath?

Let's take a moment to analyze all potential use cases and establish a solid way to resolve all links on windows.