BOSTON — Meb Keflezighi strode alone down Boylston Street, past the patched-up sidewalks and re-opened restaurants that line the path to the Boston Marathon finish line. On his race bib he had the names of those killed there by a pair of homemade bombs, and on his shoulders he would soon drape an American flag. An Eritrean by birth, Keflezighi came to the United States to escape war and poverty in his African homeland.