“Cowboys” taps directly into the myth of the American male, with his leather boots and blue jeans, square jaw and wide stance, as immortalized in the collective imagination by painter Frederick Remington, director John Ford and decades of Marlboro tobacco advertising. But it does so with a twist: This debut feature from Anna Kerrigan explores how that tough-guy archetype impresses itself on a gender-nonconforming child. Who says that cowboys have to be boys? And that girls must stay girls?