Windows: Remove use of deprecated function SHGetFolderPath and use SHGetKnownFolderPath instead

[kit-ty-kate]

Fixes #5861

Based on the 6 years old comment at the top of OPAMW_SHGetFolderPath we might probably want to use the new API and don't want to spend the time supporting such an old system

[dra27]

Thanks for this - apart from getting rid of youthfully exuberent (= daft) comments of mine from the source code, the fact SHGetKnownFolderPath handles allocation itself will automatically deal with long paths (which wasn't a thing when I wrote the original code; I just still harboured an unhealthy desire to keep old OSes working...!). I have a hunch that long paths might be what's gone wrong in the original issue (although it's surprising).

My instinct would be to have a single C stub taking a variant (more closely wrapping one API function), but actually I think your instinct is better in having separate functions! 🙂

We need to link opam with ole32 (for CoTaskMemFree) and uuid (for the FOLDERID_ "constants") - I'm pushing a commit which hopefully does that bit!

[dra27]

LGTM, thanks!