http: h1/h2 proxy unification

- use shared code for setting up the CONNECT request
  when tunneling, used in HTTP/1.x and HTTP/2 proxying
- eliminate use of Curl_buffer_send() and other manipulations
  of `data->req` or `data->state.ulbuf`

Closes #11808

lib/http1.h

@@ -56,6 +56,8 @@ ssize_t Curl_h1_req_parse_read(struct h1_req_parser *parser,
 CURLcode Curl_h1_req_dprint(const struct httpreq *req,
                             struct dynbuf *dbuf);
 
+CURLcode Curl_h1_req_write_head(struct httpreq *req, int http_minor,
+                                struct dynbuf *dbuf);
 
 #endif /* !CURL_DISABLE_HTTP */
 #endif /* HEADER_CURL_HTTP1_H */