NEW YORK (Reuters) - The ageing New York Yankees, struggling to inject youth into their roster, appear poised to sign two of their evergreen core members for one more go-round in a drive for a sixth World Series ring since 1996. Left-handed starting pitcher Andy Pettitte, 40, and all-time Major League Baseball saves leader Mariano Rivera, who turns 43 on Thursday, are reportedly on the verge of signing deals for one last campaign. Those pitchers could well be effective in 2013. Pettitte returned from a one-year retirement last season to go 5-4 with an impressive 2. ...