* France absent after row over Kagame remarks
* Rebel-turned-statesman Kagame rebuilt nation
* Genocide in 1994 killed 800,000 in 100 days
By Jenny Clover
KIGALI, April 7 (Reuters) - The United Nations chief told a packed stadium of sombre and weeping Rwandans on Monday the world would "never again" let genocide tear their nation apart, at a ceremony marking 20 years since 800,000 ...