DOCTOR DETROIT (1983) **1/2 Dan Aykroyd, Howard Hesseman, Donna Dixon, Fran Drescher, TK Carter, George Furth, James Brown. Aykroyd has a field day as milquetoast college professor who unwittingly is enlisted by a pimp to assume the identity of a ganglord mack daddy as the eponymous not-to-be-trifled man about town. Frequently funny especially his tete a tete with his archenemy, Mom, in a junk yard: `Mom, I'm gonna rip off your head and s**t down your neck!' Brown's appearance livens things up with a neat, goofy dance spotlight for the antic Aykroyd