tidy-up: assortment of small fixes

- examples/headerapi: fix wrong cast.
- curl_ngtcp2: delete stray character from error message.
- rustls: fix inline variable declaration.
- sendf: drop redundant `int` cast.
- libtest/cli_ws_data: drop cast with mismatched signedness.

Cherry-picked from #18343

Closes #18664

lib/sendf.c

@@ -857,7 +857,7 @@ static CURLcode cr_in_rewind(struct Curl_easy *data,
     Curl_set_in_callback(data, FALSE);
     CURL_TRC_READ(data, "cr_in, rewind via set.seek_func -> %d", err);
     if(err) {
-      failf(data, "seek callback returned error %d", (int)err);
+      failf(data, "seek callback returned error %d", err);
       return CURLE_SEND_FAIL_REWIND;
     }
   }