MACHINEGUN FIRE on city streets in broad daylight, pushers arrested in schoolyard playgrounds, commando operations against drug lords in South American jungles—such are the images that in recent years have frightened the public and galvanized it to declare a war on drugs.  Proposals for coping with drug-related crimes range from executing “kingpins” to banning beepers from public schools. Legislation enacted in 1988 provides more federal money for drug treatment and education, and denies certain benefits (including public housing) to drug felons. Former Mayor Koch called for using the military to arrest drug offenders and for incarcerating them in tents in the Nevada desert. Others urge legalization of narcotic use. The nation now has a drug “’czar,” who has proposed an elaborate national strategy that includes expanding drug-testing programs, sending offenders to boot camps, and publicizing the names of first-time users.