SINGAPORE, June 4 (Reuters) - Senior officials from about two dozen of the world's major intelligence agencies held a secret meeting on the fringes of the Shangri-La Dialogue security meeting in Singapore this weekend, five people told Reuters. "There is an unspoken code among intelligence services that they can talk when more formal and open diplomacy is harder - it is a very important factor during times of tension, and the Singapore event helps promote that. See here for a complete list of exchanges and delays