lib/model, lib/scanner: Properly ignore symlinks on Windows (fixes #4035)

Adds a unit test to ensure we don't scan symlinks on Windows. For the
rwfolder, trusts that the logic in the invalid check is correct and that
the check is actually called from the need loop.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4042

lib/scanner/walk.go

@@ -373,6 +373,12 @@ func (w *walker) walkDir(relPath string, info os.FileInfo, dchan chan protocol.F
 // walkSymlink returns nil or an error, if the error is of the nature that
 // it should stop the entire walk.
 func (w *walker) walkSymlink(absPath, relPath string, dchan chan protocol.FileInfo) error {
+	// Symlinks are not supported on Windows. We ignore instead of returning
+	// an error.
+	if runtime.GOOS == "windows" {
+		return nil
+	}
+
 	// We always rehash symlinks as they have no modtime or
 	// permissions. We check if they point to the old target by
 	// checking that their existing blocks match with the blocks in