improve query/form handling (tlsuv_http_req_query/tlsuv_http_req_form)

[ekoby]

• make repeated _query/_form calls additive up to the max limit
• handle query correctly if request path contains query parameters already

   req = tlsuv_http_req(clt, "GET", "/path?q=test", data);
   tlsuv_http_req_query(req, 1, &(tlsuv_http_pair){ "foo", "bar" });
   tlsuv_http_req_query(req, 1, &(tlsuv_http_pair){ "bar", "baz" });

should make request GET /path?q=test&foo=bar&bar=baz