localtime: detect thread-safe alternatives and use them

- add local API `toolx_localtime()` to wrap the banned function
  `localtime()`. Used from libcurl, libtests and test servers.
- auto-detect and use `localtime_r()` where available (e.g. Linux).
  Also to support multi-threading.
- use `localtime_s()` on Windows. It requires MSVC or mingw-w64 v4+.
  Also to support multi-threading.
  Use local workaround to also support mingw-w64 v3.
- add `src/toolx` to keep internal APIs used by the curl tool and tests,
  but not by libcurl. `toolx_localtime()` is the first API in it.
- replace `localtime()` calls with `toolx_localtime()`.
  Except in examples.
- note Windows XP's default `msvcrt.dll` doesn't offer secure CRT APIs.
  XP likely needs a newer version of this DLL, or may not run.
- note that `localtime()` mirrors `gmtime()`, with the difference that
  `gmtime()`'s internal wrapper lives in curlx.

Also:
- drop redundant `int` casts.

Refs:
https://learn.microsoft.com/cpp/c-runtime-library/reference/localtime-localtime32-localtime64
https://learn.microsoft.com/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
https://pubs.opengroup.org/onlinepubs/9799919799/functions/localtime.html
https://linux.die.net/man/3/localtime_r

Ref: #19955 (for `gmtime_r()`)
Follow-up to 54d9f060b4
Closes #19957

src/toolx/tool_time.c

@@ -0,0 +1,61 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+#include "tool_time.h"
+
+#if defined(__MINGW32__) && (__MINGW64_VERSION_MAJOR <= 3)
+#include <sec_api/time_s.h>  /* for _localtime32_s(), _localtime64_s() */
+#ifdef _USE_32BIT_TIME_T
+#define localtime_s _localtime32_s
+#else
+#define localtime_s _localtime64_s
+#endif
+#endif
+
+/*
+ * toolx_localtime() is a localtime() replacement for portability. Do not use
+ * the localtime_s(), localtime_r() or localtime() functions anywhere else but
+ * here.
+ */
+CURLcode toolx_localtime(time_t intime, struct tm *store)
+{
+#ifdef _WIN32
+  if(localtime_s(store, &intime)) /* thread-safe */
+    return CURLE_BAD_FUNCTION_ARGUMENT;
+#elif defined(HAVE_LOCALTIME_R)
+  const struct tm *tm;
+  tm = localtime_r(&intime, store); /* thread-safe */
+  if(!tm)
+    return CURLE_BAD_FUNCTION_ARGUMENT;
+#else
+  const struct tm *tm;
+  /* !checksrc! disable BANNEDFUNC 1 */
+  tm = localtime(&intime); /* not thread-safe */
+  if(tm)
+    *store = *tm; /* copy the pointed struct to the local copy */
+  else
+    return CURLE_BAD_FUNCTION_ARGUMENT;
+#endif
+
+  return CURLE_OK;
+}

src/toolx/tool_time.h

@@ -0,0 +1,30 @@
+#ifndef HEADER_TOOLX_TOOL_TIME_H
+#define HEADER_TOOLX_TOOL_TIME_H
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+#include "curl_setup.h"
+
+CURLcode toolx_localtime(time_t intime, struct tm *store);
+
+#endif /* HEADER_TOOLX_TOOL_TIME_H */