lib/fs: Handle deduplicated files on NTFS (fixes #1845)

These files always have the symlink bit set, because they are reparse
points. Nonetheless they are not symlinks, and Lstat reports a size for
them. We use this fact to disambiguate, and hope fervently that nothing
else matches this description so it comes back to bite us...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4622
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