After a wimpy winter, spring break has arrived, and skiers and snowboarders from Maine to Mammoth Lakes are stoked. More than 18 feet of snow has dropped at Squaw Valley, Calif., in March; Utah’s famed powder resorts have finally broken the 100-inch mark; and New England has been pummeled by four big storms pushing closing dates to late April. At the same time, there are warning signs about the future of the sport.