WASHINGTON (Reuters) - President Barack Obama and Afghan President Hamid Karzai will hold a critical round of talks on Friday that could help determine how fast the United States withdraws troops from Afghanistan and whether it leaves a residual force after 2014. Hosting Karzai at the White House, Obama faces the challenge of pressing ahead with his re-election pledge to continue winding down the long war in Afghanistan while preparing the Afghan government to prevent a slide back into chaos and a Taliban resurgence once most NATO forces are gone. ...