TAPACHULA, Chiapas— In a rundown neighborhood near the Mexico-Guatemala border lies the Bélen shelter, a house that assists what are mostly Central American migrants who are beginning their journey up through Mexico to the United States. The shelter is managed by Flor Maria Rigoni, a 71-year-old Italian priest who carries a big crucifix tucked into his waistband is if it were a gun. His long white beard, flowing robe and beatdown sandals draw stares wherever