Palm oil is common in U.S. diets, yet it lives in obscurity. Big food manufacturers add it to a wide array of packaged foods — it is in Skittles, Oreos and Milky Ways, for example. But the name may be familiar only to those who read labels. Now, though, palm oil is getting attention, much of it unwanted. One of the world’s largest palm oil producers is a Malaysian company dogged by allegations that it subjects migrant workers to forced labor, and it is facing legal petitions from groups that want customs officials to block the company’s palm oil from entering the United States.