Internet Emerging As Potent Terrorist Tool By THOMAS WAGNER     LONDON (AP) -- The images coming out of the latest hostage crisis in Iraq - capped by dramatic video of British captive Kenneth Bigley begging for his life - have transfixed Britons, left governments looking helpless, and revived a classic dilemma about whether to negotiate with terrorists.    But the plight of the British construction worker and his two murdered American colleagues has also raised new concerns about terrorists' tremendous ability to set agendas in an Internet age that makes their messages - even in the form of shocking beheading videos - all but impossible to stop...