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.go

@@ -0,0 +1,22 @@
+// +build !windows,!plan9,!js
+
+package local
+
+import (
+	"os"
+	"syscall"
+)
+
+// isCircularSymlinkError checks if the current error code is because of a circular symlink
+func isCircularSymlinkError(err error) bool {
+	if err != nil {
+		if newerr, ok := err.(*os.PathError); ok {
+			if errcode, ok := newerr.Err.(syscall.Errno); ok {
+				if errcode == syscall.ELOOP {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}