Kitchens here were once flush with emblematic dishes — chowder, baked beans, fried whole belly clams — elemental foods prepared for generations in the same way. Not anymore. These days a trip along the coast in search of real cooking is too often met with overcooked lobsters, fries from the freezer, and chowder as thick as mashed potatoes. But times are changing. At Eventide, a new oyster bar in the Old Port here, the good Maine food is once again made from scratch.