BEIRUT/AMMAN (Reuters) - Mystery surrounded the whereabouts of Syrian President Bashar al-Assad on Thursday, a day after a bomber killed and wounded his security chiefs and rebels closed in on the centre of Damascus, vowing to "liberate" the capital. The Syrian leader made no public appearance and no statement after a bomber killed his powerful brother-in-law, his defense minister and a top general. By the early hours of Thursday, residents had reported no let-up in the heaviest fighting to hit the capital in a 16-month revolt against Assad's rule. ...