LONDON (Reuters) - After an infamous tiff between Elton John and Madonna over lip-synching, Britain's Musicians' Union has called on performers to come clean -- audiences should be told if they are miming rather than singing. The union is urging promoters, producers and artists to back its campaign for lip-synching to be clearly labeled during TV shows, in pop arenas and on stage. "Stand up and be honest about it. We won't knock you for using recordings," said union spokesman Keith Ames, wearied by the sight of bands with miming singers backed by guitarists going through the motions to a...