THE LIFE INSURANCE GAME By Ronald Kessler Holt, Rinehart and Winston 289 pp. $16.95 NO MATTER HOW — OR HOW LONG — you look at life insurance, the laws of averages are not encouraging. If you let an agent inside your home, the chances are 1 in 2 you will wind up buying a policy. The chances are 4 in 5 you will buy