By Marwa Rashad RIYADH, Oct 20 (Reuters) - Saudi Arabia's biggest bank has responded to criticism of its operations from Islamic scholars by pledging to convert itself into a full-fledged Islamic bank within about five years. The decision, made as the bank launches a $6 billion initial public offer of its shares, the largest-ever equity sale in the Arab world, shows how Saudi Arabia's conservative brand of Islam can have a big impact on business decisions in the kingdom. ...