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/http_proxy.h

@@ -30,6 +30,15 @@
 
 #include "urldata.h"
 
+CURLcode Curl_http_proxy_get_destination(struct Curl_cfilter *cf,
+                                         const char **phostname,
+                                         int *pport, bool *pipv6_ip);
+
+CURLcode Curl_http_proxy_create_CONNECT(struct httpreq **preq,
+                                        struct Curl_cfilter *cf,
+                                        struct Curl_easy *data,
+                                        int http_version_major);
+
 /* Default proxy timeout in milliseconds */
 #define PROXY_TIMEOUT (3600*1000)