MANAMA, Jan 5 (Reuters) - The head of the Bahrain's main Shi'ite Muslim opposition group was remanded in custody for a further 15 days on Monday in a move that could further inflame sectarian tensions in the Kingdom. Sheikh Ali Salman, head of the al-Wefaq Islamic Society, was arrested on Dec. 28 after leading a protest rally against elections in **vember which his party boycotted. Washington, a close ally of Bahrain which bases its Fifth Fleet there, has already expressed deep concern over his detention. ...