A Multicentre, Randomised, Double-blind, Parallel Group, Placebo-controlled Study to Investigate the Long-term Effects of Salmeterol/Fluticasone Propionate (Seretide tm) 50/500mcg BD, Salmeterol 50mcg BD and Fluticasone Propionate 500mcg BD, All Delivered Via the Diskus tm/Accuhaler tm Inhaler, on Mortality and Morbidity of Subjects With Chronic Obstructive Pulmonary Disease (COPD) Over 3 Years of Treatment