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/headers.h

@@ -36,6 +36,12 @@ struct Curl_header_store {
   char buffer[1]; /* this is the raw header blob */
 };
 
+/*
+ * Initialize header collecting for a transfer.
+ * Will add a client writer that catches CLIENTWRITE_HEADER writes.
+ */
+CURLcode Curl_headers_init(struct Curl_easy *data);
+
 /*
  * Curl_headers_push() gets passed a full header to store.
  */
@@ -48,6 +54,7 @@ CURLcode Curl_headers_push(struct Curl_easy *data, const char *header,
 CURLcode Curl_headers_cleanup(struct Curl_easy *data);
 
 #else
+#define Curl_headers_init(x) CURLE_OK
 #define Curl_headers_push(x,y,z) CURLE_OK
 #define Curl_headers_cleanup(x) Curl_nop_stmt
 #endif