lib: replace _tcsncpy/wcsncpy/wcscpy with _s counterparts (Windows)

[vszakats]

Replace:

• curl_sspi: macro _tcsncpy() with _tcsncpy_s().
• curlx/fopen: wcsncpy() with wcsncpy_s().
• curlx/fopen: wcscpy() with wcscpy_s().

Use of the pre-existing functions were safe. This patch aims to use the
recommended Windows CRT functions. Handle errors returned by them. Also
to avoid the compiler warnings silenced via _CRT_SECURE_NO_WARNINGS:

lib/curl_sspi.c(152): warning C4996: 'wcsncpy': This function or variable may be unsafe. Consider using wcsncpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS.
lib/curlx/fopen.c(161): warning C4996: 'wcsncpy': This function or variable may be unsafe. Consider using wcsncpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS.
lib/curlx/fopen.c(162): warning C4996: 'wcscpy': This function or variable may be unsafe. Consider using wcscpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS.
lib/curlx/fopen.c(174): warning C4996: 'wcsncpy': This function or variable may be unsafe. Consider using wcsncpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS.
lib/curlx/fopen.c(175): warning C4996: 'wcscpy': This function or variable may be unsafe. Consider using wcscpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS.

Refs:
https://learn.microsoft.com/cpp/c-runtime-library/reference/strncpy-strncpy-l-wcsncpy-wcsncpy-l-mbsncpy-mbsncpy-l
https://learn.microsoft.com/cpp/c-runtime-library/reference/strncpy-s-strncpy-s-l-wcsncpy-s-wcsncpy-s-l-mbsncpy-s-mbsncpy-s-l
https://learn.microsoft.com/cpp/c-runtime-library/security-features-in-the-crt

Cherry-picked from #19581 (in part)

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• replacement functions return an error code. Check them?