cmd: Use FormatMessage() for better internationalization support.

[Frédéric Delanoy]

On Tue, Dec 6, 2011 at 17:40, Francois Gouget <fgouget at free.fr> wrote:
> This makes it possible to reorder the format string placeholders in message translations.
> Also add a WCMD_format_string() function to mirror the WCMD_output*() ones and so all resources can use the same formatting style.
> This also simplifies the padding code for the 'dir /w' command.

> +            static const WCHAR empty[] = {'\0'};

There's already a suitable nullW string available