Revert d259429

As reported on irc:

```
17:51 <q> jvoisin, fortify-headers seems to be broken (on Alpine at least)
17:52 <q> Repeating the message from over-there:
17:52 <q> /usr/include/fortify/string.h: In function 'strncat':
17:52 <q> /usr/include/fortify/string.h:297:36: error: implicit declaration of function 'strnlen'; did you mean 'strlen'? [-Wimplicit-function-declaration]
17:52 <q> This is with a simple file that includes string.h and call strncat, built with c99 -O1 f.c
```

Author: jvoisin

include/string.h

@@ -208,7 +208,9 @@ _FORTIFY_FN(stpncpy) char *stpncpy(char * _FORTIFY_POS0 __d, const char *__s,
 #if __has_builtin(__builtin___stpncpy_chk) && FORTIFY_USE_NATIVE_CHK
 	return __builtin___stpncpy_chk(__d, __s, __n, __fh_bos(__d, 0));
 #else
-	__fh_size_t max_len_s = strnlen(__s, __n);
+	__fh_size_t __max_len_s = strlen(__s);
+	if (max_len_s > __n)
+		max_len_s = __n;
 	if (__fh_overlap(__d, max_len_s, __s, max_len_s))
 		__builtin_trap();