* Boko Haram major security challenge in Nigeria
* Sect killed 20 in raids in northeast stronghold
* President sending more troops to fight insurgents
By Lanre Ola
MAIDUGURI, Nigeria, Sept 7 (Reuters) - The Nigerian army said on Saturday it had tracked down and killed 50 members of Boko Haram, days after the Islamist sect was blamed for killing 20 villagers in raids in its northeastern ...