MIAMI — Cuba has loosened a policy banning Cuban-born people from arriving by sea, allowing Carnival Corp. to go forward with the first U.S. cruise to the island in a half-century, the Cuban government and the Miami-based cruise line announced today.The company at first barred Cuban-born Americans from buying tickets for the planned May 1 cruise to comply with Cuba’s ban, drawing complaints from the Cuban-American community in Miami and a discrimination lawsuit. Then, the company said it would sell tickets to …