UnitedHealth to Buy Definity Health  CHICAGO (Reuters) - UnitedHealth Group Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=UNH.N target=/stocks/quickinfo/fullquote"&gt;UNH.N&lt;/A&gt; on  Monday said it would buy Definity Health Corp. for \$300  million, as the No. 1 U.S. health insurer seeks to offer more  health savings accounts, which give consumers incentives to  rein in health costs.