Make net/http/pprof location agnostic.

Make it possible to place the pprof endpoints at a location
different from /debug/pprof.
I think we should now change Index to:

    func Index(w http.ResponseWriter, req *http.Request) {
        IndexAtRoot("/debug/pprof/").ServeHTTP(w, req)
    }

Then we won't be duplicating its logic.