On Windows, C11 localtime_r() and gmtime_r() functions are not
available. While localtime() and gmtime() functions are already thread
safe because they use Thread Local Storage, it's quite ugly to #ifdef
around every localtime_r() and gmtime_r() usage to make the usage also
thread-safe on POSIX platforms.
The commit adds wrappers around Windows localtime_s() and gmtime_s()
functions.
NOTE: The implementation of localtime_s and gmtime_s in Microsoft CRT
are incompatible with the C standard since it has reversed parameter
order and errno_t return type.
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