Russia has taken another major step toward restricting its once freewheeling Internet, as President Vladimir V. Putin quietly signed a new law requiring popular online voices to register with the government, a measure that lawyers, Internet pioneers and political activists said Tuesday would give the government a much wider ability to track who said what online. Mr. Putin’s action on Monday, just weeks after he disparaged the Internet as “a special C.I.A. project,” borrowed a page from the restrictive Internet playbooks of many governments around the world that have been steadily smothering online freedoms they once tolerated.