ON A PLANE OVER UPPER NILE STATE, South Sudan — Swooping low over the banks of a Nile River tributary, an aid flight run by retired American military officers released a stream of food-stuffed sacks over a town emptied by fighting in South Sudan , a country wracked by conflict . Last week's air drop was the latest in a controversial development: private contracting firms led by former U.S. intelligence officers and military veterans delivering aid to some of the world's deadliest conflict zones,