The government is taking steps to curb use of some long-acting asthma drugs taken by millions, issuing safety restrictions to lower an uncommon but potentially life-threatening risk that asthma could worsen suddenly. The warnings cover the drugs Advair, Symbicort, Foradil and Serevent. The FDA said they should be used only by asthmatics who can’t control their disease with other drugs — and then only for the shortest time possible.