By Lanre Ola MAIDUGURI, Nigeria (Reuters) - Nigerian soldiers have tracked down and killed 50 members of the Islamist sect Boko Haram in its northeastern stronghold, an army spokesman said on Saturday. Army units mounted the operation after suspected Boko Haram fighters killed 20 people in two attacks on Wednesday and Thursday in villages in northeastern Borno state. "Troops pursued the terrorists to their camps and with air support about 50 terrorists were killed in a shoot-out," army spokesman Sagir Musa told reporters in Maiduguri. ...