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  XP likely needs a newer version of this DLL, or may not run.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

tests/server/util.c

@@ -23,6 +23,8 @@
  ***************************************************************************/
 #include "first.h"
 
+#include <toolx/tool_time.h>
+
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>
 #endif
@@ -83,8 +85,9 @@ void logmsg(const char *msg, ...)
   char buffer[2048 + 1];
   FILE *logfp;
   struct curltime tv;
+  CURLcode result;
   time_t sec;
-  struct tm *now;
+  struct tm now;
   char timebuf[50];
   static time_t epoch_offset;
   static int    known_offset;
@@ -100,12 +103,12 @@ void logmsg(const char *msg, ...)
     known_offset = 1;
   }
   sec = epoch_offset + tv.tv_sec;
-  /* !checksrc! disable BANNEDFUNC 1 */
-  now = localtime(&sec); /* not thread safe but we do not care */
+  result = toolx_localtime(sec, &now);
+  if(result)
+    memset(&now, 0, sizeof(now));
 
   snprintf(timebuf, sizeof(timebuf), "%02d:%02d:%02d.%06ld",
-           (int)now->tm_hour, (int)now->tm_min, (int)now->tm_sec,
-           (long)tv.tv_usec);
+           now.tm_hour, now.tm_min, now.tm_sec, (long)tv.tv_usec);
 
   va_start(ap, msg);
 #ifdef __clang__