Whose Life Is It, Anyway? began as a one-hour British TV drama, which treated the ''right to die'' subject more like an abstract issue than a flesh-and-blood dilemma. Fleshed out, it subsequently became a hit play in London and on Broadway. The movie version (rated PG for some nudity and vulgar language) has been effectively directed by John Badham, and Richard Dreyfuss plays the central role with a good deal of humor and not too many of his usual mannerisms. Yet there is something perverse about a movie that wants us to root for someone's decision to stop living. And the film does want us to cheer, building to its courtroom-type climax with all the dramatic tricks and Hollywood gimmicks at its disposal. No matter how well-meaning the attempt may be, such hugely complex and ineffably sad issues deserve a kind of care and thought that just aren't part of the entertainment-film arsenal.