* Days of shelling kill at least six civilians
* Congo accuses Rwanda of sending troops to support rebels
* Kigali accuses Congo army of shelling Rwandan villages
By Kenny Katombe
GOMA, Democratic Republic of Congo, Aug 24 (Reuters) - Shells fired by M23 rebels killed at least three people in Congo's eastern city of Goma on Saturday, the United Nations said, as Congo and Rwanda traded ...