http: add support to read and store the referrer header

- add CURLINFO_REFERER libcurl option
- add --write-out '%{referer}' command-line option
- extend --xattr command-line option to fill user.xdg.referrer.url extended
  attribute with the referrer (if there was any)

Closes #6591

lib/getinfo.c

@@ -145,6 +145,10 @@ static CURLcode getinfo_char(struct Curl_easy *data, CURLINFO info,
        option had been enabled! */
     *param_charp = data->info.wouldredirect;
     break;
+  case CURLINFO_REFERER:
+    /* Return the referrer header for this request, or NULL if unset */
+    *param_charp = data->change.referer;
+    break;
   case CURLINFO_PRIMARY_IP:
     /* Return the ip address of the most recent (primary) connection */
     *param_charp = data->info.conn_primary_ip;