But who, pray, is Detective Chief Inspector Reginald Wexford, and where on earth is Kings Markham? These answers will be found on Arts & Entertainment tomorrow night at 10, when the first hour of a three-part dramatization of Ruth Rendell's ''Shake Hands Forever'' is presented on the cable network's new ''Masters of Mystery'' series. (The subsequent episodes will be shown Feb. 12 and 19, in the same Monday-night slot.) ''Surely you Americans have heard of Wexford,'' Miss Rendell said, sounding slightly aghast in a recent telephone conversation from her home in the Suffolk district of England.