http: enable haproxy support for hyper backend

This is done by having native code do the haproxy header output before
hyper issues its request. The little downside with this approach is that
we need the entire Curl_buffer_send() function built, which is otherwise
not used for hyper builds.

If hyper ends up getting native support for the haproxy protocols we can
backpedal on this.

Enables test 1455 and 1456

Closes #8034

lib/http.h

@@ -54,15 +54,11 @@ char *Curl_copy_header_value(const char *header);
 char *Curl_checkProxyheaders(struct Curl_easy *data,
                              const struct connectdata *conn,
                              const char *thisheader);
-#ifndef USE_HYPER
 CURLcode Curl_buffer_send(struct dynbuf *in,
                           struct Curl_easy *data,
                           curl_off_t *bytes_written,
                           curl_off_t included_body_bytes,
                           int socketindex);
-#else
-#define Curl_buffer_send(a,b,c,d,e) CURLE_OK
-#endif
 
 CURLcode Curl_add_timecondition(struct Curl_easy *data,
 #ifndef USE_HYPER