In yet another sign of a strengthening recovery, the United States added 227,000 net jobs in February, the third consecutive month of gains over 200,000. The unemployment rate was unchanged from 8.3 percent in January, the Labor Department reported Friday, as nearly a half million people who had been staying on the sidelines rejoined the search for work. The strong job growth numbers could bolster President Obama’s effort to make the case to voters that his economic policies are working.