UPDATE 1 — Costa Rica's Grynspan narrowly leads informal UN chief poll, sources say
(Adds details starting in paragraph 2) UNITED NATIONS, Sept 18 (Reuters) — An informal poll by the United Nations Security Council on Friday placed former Costa Rican Vice President Rebeca Grynspan as the narrow frontrunner in the contest to become the next U.N. secretary-general, diplomats and other sources close to t