Replace the single strnlen() use with strlen() so as not to require
something from posix.1 (2008) when we supposedly only require c99.
If someone manages to produce a string that's longer than will fit
within size_t, strlen()'s length count will wrap and Strlen_() might
not notice. Since size_t has to be at least 32 bits, that doesn't
seem like something to worry about.
If checking for size_t overflow is considered essential, we should
just switch to counting the length via an in-line loop that imposes
a similar limit to strnlen().
7c1512fa99