Why couldn't "The Sessions" stay on the course it sets out on? John Hawkes gives a star-making performance as real-life writer Mark O'Brien, a man who contracted polio as a child and has been held immobile by weak muscles and an inability to breathe for too long without help from an iron lung. William H. Macy does terrific work as a priest who Mark confides in about his sexual awakening. This happens to coincide with