US Airways Seeks Concessions from Unions  WASHINGTON (Reuters) - Bankrupt US Airways &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=UAIRQ.OB target=/stocks/quickinfo/fullquote"&gt;UAIRQ.OB&lt;/A&gt;  sought consent from its big unions on Wednesday for temporary  relief from contracts and threatened to go to court to force  changes if workers did not voluntarily agree.