Women's rights groups on Friday demanded an international investigation into the killing of a Libyan activist who had become a symbol of her country's efforts to create a democracy.   The shooting and stabbing of Salwa Bugaighis shortly after she cast her vote in Libya's parliamentary elections last year shocked the international community, with U.S. National Security Adviser Susan Rice calling the assassination "brutal and senseless." After Gadhafi was overthrown, Bugaighis became one of the most outspoken voices against militia fighters and Islamic extremists. She also was deputy