As the city is getting ready for Sail Boston, one young sailor is across town in a hospital, fighting for his life. Trevor O’Neil would rather be on the USS Constitution, getting ready to welcome crowds. “I have Stage 4 esophageal cancer,” O’Neil said. A few months ago, the 21-year-old sailor was doing what he loves, hanging in the rigs aboard one of the Navy’s most historic ships. “When I look at that picture, it just is the epitome of who he i…