http: add Hijacker type; remove Hijack from ResponseWriter

http: add Hijacker type; remove Hijack from ResponseWriter

The Hijack functionality wasn't removed, but now you have
to test if your ResponseWriter is also a Hijacker:

func ServeHTTP(rw http.ResponseWriter, req *http.Request) {
    if hj, ok := rw.(http.Hijacker); ok {
        hj.Hijack(..)
    }
}

R=rsc
CC=golang-dev
https://golang.org/cl/4245064