udpkg should probably use libd-i4

[Petter Reinholdtsen <pere@hungry.com>]

Good work adding more error detection in udpkg.  I look forward to
test it in a low-memory situation. :)

[Matt Kraai]
> -					SYSTEM(buf);
> +					if (SYSTEM(buf) != 0)

I suspect it would be better to use the logging system() wrapper in
libd-i4.  That way, it would be easier to see the messages when
something fail.