By David Brunnstrom WASHINGTON (Reuters) -Economic sanctions, the primary means the United States has used for years to try to exert pressure on North Korea, have abjectly failed to halt its nuclear and missile programs or to bring the reclusive northeast Asian state back to the negotiating table. Instead, North Korea’s ballistic missile program has become stronger and it has carried out a record-breaking testing regime of multiple types of weapons this year – including of intercontinental ballistic missiles designed to reach the U.S. mainland. Expectations are that it may soon end a self-impo…