win32: use NT-prefixed "\\?\" paths

win32: use NT-prefixed "\\?\" paths

When turning UTF-8 paths into UCS-2 paths for Windows, always use
the \\?\-prefixed paths.  Because this bypasses the system's
path canonicalization, handle the canonicalization functions ourselves.

We must:
 1. always use a backslash as a directory separator
 2. only use a single backslash between directories
 3. not rely on the system to translate "." and ".." in paths
 4. remove trailing backslashes, except at the drive root (C:\)