By Matt Spetalnick and Laura MacInnis NEW YORK (Reuters) - President Barack Obama called on Tuesday for the last of Muammar Gaddafi's loyalist forces to lay down their arms as he announced the return of the U.S. ambassador to Tripoli and pledged to help Libya rebuild. "Today, the Libyan people are writing a new chapter in the life of their nation," Obama said in prepared remarks for a high-level U.N.