request: eliminate request getheader bool, reverse header default

Deduce that the transfer response expects headers by the protocol
handler implementing `write_resp_hd` callback. This eleminates the
`getheader` parameter in the `Curl_xfer_setup_*()` methods.

Add an implementation to RTSP for `write_resp_hd`, joining the HTTP
protocol in the only handlers having it.

Reverse the default of request's `header` bit that signals that headers
are expected. Default is now FALSE, set to TRUE when setting up the
transfer by presence of `write_resp_hd` in the protocol handler.

Closes #18218

lib/http.c

@@ -2415,7 +2415,7 @@ static CURLcode http_req_complete(struct Curl_easy *data,
 out:
   if(!result) {
     /* setup variables for the upcoming transfer */
-    Curl_xfer_setup_sendrecv(data, FIRSTSOCKET, -1, TRUE);
+    Curl_xfer_setup_sendrecv(data, FIRSTSOCKET, -1);
   }
   return result;
 }