* Security situation hinders aid agencies
* MSF pares back operations at airport after shootings
* Some 12,000 Chadians have returned home in recent days
By Paul-Marin Ngoupana
BANGUI, Jan 3 (Reuters) - Violence in Central African Republic has uprooted nearly a million people, a fifth of the population, and is hampering aid efforts, particularly in the capital Bangui, the U.N. refugee ...