Thirty years ago, Armando Mejia fled war-torn El Salvador and sneaked into the United States. He was 17, with a sixth-grade education and two dollars in his pocket. For the next two decades, he toiled in the kitchens of Washington-area restaurants, working his way up from dishwasher to chef. Today Mejia, 49, owns three upscale Tex-Mex bistros in Northern Virginia, and a fourth in the District. A shrewd and genial host, he serves perfect frozen margaritas and supports local sc