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  XP likely needs a newer version of this DLL, or may not run.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

@@ -27,6 +27,7 @@
 #include "tool_msgs.h"
 #include "tool_cb_dbg.h"
 #include "tool_util.h"
+#include "toolx/tool_time.h"
 
 static void dump(const char *timebuf, const char *idsbuf, const char *text,
                  FILE *stream, const unsigned char *ptr, size_t size,
@@ -34,7 +35,6 @@ static void dump(const char *timebuf, const char *idsbuf, const char *text,
 
 /*
  * Return the formatted HH:MM:SS for the tv_sec given.
- * NOT thread safe.
  */
 static const char *hms_for_sec(time_t tv_sec)
 {
@@ -42,10 +42,12 @@ static const char *hms_for_sec(time_t tv_sec)
   static char hms_buf[12];
 
   if(tv_sec != cached_tv_sec) {
-    /* !checksrc! disable BANNEDFUNC 1 */
-    struct tm *now = localtime(&tv_sec);  /* not thread safe either */
+    struct tm now;
+    CURLcode result = toolx_localtime(tv_sec, &now);
+    if(result)
+      memset(&now, 0, sizeof(now));
     curl_msnprintf(hms_buf, sizeof(hms_buf), "%02d:%02d:%02d",
-                   now->tm_hour, now->tm_min, now->tm_sec);
+                   now.tm_hour, now.tm_min, now.tm_sec);
     cached_tv_sec = tv_sec;
   }
   return hms_buf;