MADRID, February 25 (AFP) - A suspected Islamic extremist believed linked to groups that planned attacks on targets in Madrid was found dead in his prison cell in the northern Spanish city of Zaragossa after apparently hanging himself, prison sources said. They said Mustafa Zanibar, whose nationality was not revealed, was found by prison staff after apparently hanging himself with his belt. Last October top Spanish judge Baltasar Garzon ordered the arrest of about 30 people suspected of belonging to a radical Islamic group that planned to attack several buildings in the Spanish capital, including the main courthouse and Real...