local: continue listing files/folders when a circular symlink is detected

Before this change a circular symlink would cause rclone to error out from the listings.

After this change rclone will skip a circular symlink and carry on the listing,
producing an error at the end.

Fixes #4743

backend/local/symlink_other.go

@@ -0,0 +1,17 @@
+// +build windows plan9 js
+
+package local
+
+import (
+	"strings"
+)
+
+// isCircularSymlinkError checks if the current error code is because of a circular symlink
+func isCircularSymlinkError(err error) bool {
+	if err != nil {
+		if strings.Contains(err.Error(), "The name of the file cannot be resolved by the system") {
+			return true
+		}
+	}
+	return false
+}