Draft: Explicitly translate paths for Windows when generating c2p state

We need to translate Unix paths (e.g. /c/Users/...) to Windows paths (e.g. C:/Users/...) when calling from a MinGW process into a native Windows process. MinGW handles this conversion automatically in simple cases (e.g. single command-line arguments), but fails in more complex cases which is why we need an explicit translation there. On Unix-like systems the translation reduces to a simple no-op (see NATIVEPATH=echo).

Marked as a draft for now since it needs more testing, especially to make sure that we e.g. don't regress on the SWI build.