Stocks backed out of early gains, turning negative in mixed trade heading into Friday's lunch hour. The Nasdaq, S&P 500 and Dow all showed fractional losses just before noon. Trade remained strong on the NYSE, up 91%, vs. a 4% decline on the Nasdaq, compared to Thursday volume. Wal-Mart (WMT) remained down 2% in about three times its average trade, keeping negative pressure on the Dow. Drugmaker Warner Chilcott (WCRX) posted the biggest move among