custom request headers

adds vmod functions curl.add_header() and curl.unset_header() to add/remove additional request headers. mostly copied from existing code.

Usage:

    sub replay_req {
        curl.add_header("X-Replay: yes");  // add custom header
        curl.add_header("Host: " + req.http.Host);  // copy Host: header from request
        curl.add_header("Accept:");  // disable libcurl's Accept: header
        curl.fetch("http://127.0.0.1" + req.url);
        curl.free();
    }