lib: always use Curl_1st_fatal instead of Curl_1st_err

Curl_1st_err() does not return the second error if the first result is
CURLE_AGAIN. This may cause errors to not become noticeable when they
should be.

Replace all use of Curl_1st_err() with Curl_1st_fatal(), which handles
CURLE_AGAIN as a not-a-real-error case.

Closes #20980

lib/http.c

@@ -4169,7 +4169,7 @@ static CURLcode http_on_response(struct Curl_easy *data,
 out:
   if(last_hd) {
     /* if not written yet, write it now */
-    result = Curl_1st_err(
+    result = Curl_1st_fatal(
       result, http_write_header(data, last_hd, last_hd_len));
   }
   if(conn_changed) {