Cal Currier didn’t know when he left Marion harbor alone on Argo that he’d be gone 28 hours, much less 28 days. The option was always there to bail, and it wasn’t a matter of bravado.             At age 16, the California-raised grandson of longtime Marion resident Bill Saltonstall knew he has his whole life in front of him and easily could have entertained a very reasonable argument to wait a year, what’s the rush, imagine how much easier this will be with that much more sailing experience. His thinking? Why not do it while he had the chance.