MOSCOW (Reuters) - U.S. President Barack Obama has held talks with Mikhail Gorbachev, a spokesman for the former Soviet leader said on Monday, in the latest sign of Washington's efforts to "press the reset button" on ties with Russia. Spokesman Konstantin Petrenko said Gorbachev met Obama and Vice President Joe Biden -- who coined the 'reset' phrase last month -- in Washington last Friday. He did not give any details of the discussion. Bilateral ties worsened sharply last year, with the two countries at odds over NATO expansion, U.S. plans for a missile shield in Eastern Europe, and Russia's war...