By Piya Sinha-Roy PARK CITY, Utah, Jan 25 (Reuters) - Porn star Linda Lovelace became a poster girl for the sexual revolution of the 1970s, but it's her story of a life marked by domestic abuse and exploitation that is the focus of a new film. "Lovelace," starring Amanda Seyfried, joined a slate of films exploring the darker side of sex and pornography at the Sundance Film Festival this week.