The wcsnlen(3) function conforms to POSIX.1-2008 and is an addition to the
ISO C standard.
size_t wcsnlen(const wchar_t *s, size_t maxlen);
The wcsnlen(3) function computes the number of wide-characters in a wide-
-string to which s points, not including NULL terminating wide-character
code and checking no more than maxlen wide-characters. This function never
examines wide-characters beyond a wide-string of maxlen size.
This function is a safer version of wcslen(3):
size_t wcslen(const wchar_t *s);
Update STANDARDS section of wmemchr(3) describing wide-character functions.