By Ivelisse Rivera and Rich McKay San Juan, Puerto Rico (Reuters) -Tropical storm Fiona gained strength on Saturday as it headed toward Puerto Rico, prompting the National Hurricane Center (NHC) to issue a hurricane warning and alerts for “life threatening floods and mudslides” for the U.S. territory and watches for the U.S. Virgin Islands. One man was found dead in the French Caribbean island of Guadeloupe, after heavy rains from the massive storm swept away his house in the Basse-Terre district, according to local authorities. The storm was about 75 miles (120 km) southeast of St Croix and a…