Result: Add from_syscall()

libluna/include/luna/Result.h

@@ -148,6 +148,13 @@ template <typename T> class Result
             return Error { error };
     }
 
+    static Result<T> from_syscall(long rc)
+    {
+        if (rc < 0) return Error { (int)-rc };
+        else
+            return (T)rc;
+    }
+
   private:
     Option<T> m_value;
     int m_error;