NAIROBI, March 18 (Reuters) - The United States has formally designated Somalia's al Shabaab militants a foreign terrorist organisation to increase pressure on what Washington says is al Qaeda's main link in the Horn of Africa nation. The al Shabaab is the militant wing of the Somalia Islamic Courts Council that took over most of southern Somalia for the second half of 2006, until Somalia's interim government and its Ethiopian military allies routed the group in a two-week war. The group, whose leader Aden Hashi Farah Ayro survived a U.S. airstrike in January 2007, is thought by security experts to...