By Gabriela Baczynska and Alastair Macdonald BRUSSELS (Reuters) - The European Union is considering a measure that would give a new EU border force powers to intervene and guard a member state's external frontier to protect the Schengen open-borders zone, EU officials and diplomats said on Friday. Such a move would be controversial. It might be blocked by states wary of surrendering sovereign control of their territory. But the discussion reflects fears that Greece's failure to manage a flood of migrants from Turkey has brought Schengen's open borders to the brink of collapse. ...