curlx/fopen: replace open CRT functions their with _s counterparts (Windows)

- `_wopen`        -> `_wsopen_s`
- `_open`, `open` -> `_sopen_s`
- `_wfopen`       -> `_wfopen_s`
- `fopen`         -> `fopen_s`
- `_wfreopen`     -> `_wfreopen_s`
- `freopen`       -> `freopen_s`

For better error handling and for using the CRT functions recommended
via warnings suppressed by `_CRT_SECURE_NO_WARNINGS`.

Also:
- add missing `freopen_s()` prototype when building with mingw-w64 <5.
  https://sourceforge.net/p/mingw-w64/mingw-w64/ci/a5d824654cdc57f6eac1bb581b078986f3eb6856/
- tests/server: replace `open()` in the signal handler with `_sopen_s()`
  on Windows.
- tests/server: reduce scope of a checksrc exception to a single line.
- checksrc: ban replaced functions.

Refs:
https://learn.microsoft.com/cpp/c-runtime-library/reference/open-wopen
https://learn.microsoft.com/cpp/c-runtime-library/reference/sopen-s-wsopen-s
https://learn.microsoft.com/cpp/c-runtime-library/reference/freopen-wfreopen
https://learn.microsoft.com/cpp/c-runtime-library/reference/fopen-wfopen
https://learn.microsoft.com/cpp/c-runtime-library/reference/fopen-s-wfopen-s
https://learn.microsoft.com/cpp/c-runtime-library/reference/freopen-s-wfreopen-s

Closes #19643

tests/server/.checksrc

@@ -6,7 +6,6 @@ allowfunc accept
 allowfunc fprintf
 allowfunc freeaddrinfo
 allowfunc getaddrinfo
-allowfunc open
 allowfunc printf
 allowfunc recv
 allowfunc send

tests/server/util.c

@@ -27,6 +27,10 @@
 #include <fcntl.h>
 #endif
 
+#ifdef _WIN32
+#include <share.h>
+#endif
+
 /* This function returns a pointer to STATIC memory. It converts the given
  * binary lump to a hex formatted string usable for output in logs or
  * whatever.
@@ -359,13 +363,17 @@ static void exit_signal_handler(int signum)
     (void)!write(STDERR_FILENO, msg, sizeof(msg) - 1);
   }
   else {
+    int fd = -1;
 #ifdef _WIN32
-#define OPENMODE S_IREAD | S_IWRITE
+    if(!_sopen_s(&fd, serverlogfile, O_WRONLY | O_CREAT | O_APPEND,
+                 _SH_DENYNO, S_IREAD | S_IWRITE) &&
+       fd != -1) {
 #else
-#define OPENMODE S_IRUSR | S_IWUSR
-#endif
-    int fd = open(serverlogfile, O_WRONLY | O_CREAT | O_APPEND, OPENMODE);
+    /* !checksrc! disable BANNEDFUNC 1 */
+    fd = open(serverlogfile,
+              O_WRONLY | O_CREAT | O_APPEND, S_IRUSR | S_IWUSR);
     if(fd != -1) {
+#endif
       static const char msg[] = "exit_signal_handler: called\n";
       (void)!write(fd, msg, sizeof(msg) - 1);
       close(fd);