http: move headers collecting to writer

- add a client writer that does "push" response
  headers written to the client if the headers api
  is enabled
- remove special handling in sendf.c
- needs to be installed very early on connection
  setup to catch CONNECT response headers

Closes #12880

lib/http.c

@@ -73,6 +73,7 @@
 #include "hostip.h"
 #include "dynhds.h"
 #include "http.h"
+#include "headers.h"
 #include "select.h"
 #include "parsedate.h" /* for the week day and month names */
 #include "strtoofft.h"
@@ -3131,6 +3132,13 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
     break;
   }
 
+  /* Add collecting of headers written to client. For a new connection,
+   * we might have done that already, but reuse
+   * or multiplex needs it here as well. */
+  result = Curl_headers_init(data);
+  if(result)
+    goto fail;
+
   http = data->req.p.http;
   DEBUGASSERT(http);