BZ #13908 Fix mktemp attribute and comment

[Nix]

On 4 Apr 2012, Andreas Jaeger uttered the following:

> -   Returns TEMPLATE, or a null pointer if it cannot get a unique file 
> name.  */
> -extern char *mktemp (char *__template) __THROW __nonnull ((1)) __wur;
> +   Returns always TEMPLATE, it's either a temporary file name or a null
> +   string if it cannot get a unique file name.  */

"Always returns".

-- 
NULL && (void)