This ancient Cambodian grilled chicken recipe, called Mann Oeng K’tem Sor, may look complicated, but it’s really just a series of simple steps.       The stone walls of the Bayon temple in Cambodia, built at the end of the 12th A.D, includes an amazing series of bas-relief pictures of the army supply trains, encampments, field kitchens, and some of the earliest depictions of Asian barbecue.   Specifically, you'll see chicken skewered on split sticks and grilled over pyramid-shaped fires. Nine hundred years later, you'll find the same chicken grilled exactly as it was done during the height of the Khmer empire.         This recipe may look complicated, but it's really just a series of simple steps.  Like most S.E. Asian recipes, the end result is a succulent, savory, melt-in-your-mouth delight. (Feet are optional... ;) )     For the Chicken and Marinade: 1 whole chicken (3 1/2 to 4 pounds) 5 cloves garlic, peeled and cut in half 2 tablespoons sugar 1 teaspoon coarse sea salt 1 tablespoon soy sauce 1…