path/filepath: make Join handle UNC paths on Windows
Unless the first element is a Universal Naming Convention (UNC)[0]
path, Join shouldn't create a UNC path on Windows.
For example, Join inadvertently creates a UNC path on Windows when
told to join at least three non-empty path elements, where the first
element is `\` or `/`.
This CL prevents creation of a UNC path prefix when the first path
element isn't a UNC path.
Since this introduces some amount of Windows-specific logic, Join is
moved to a per GOOS implementation.