dhcp: Workaround busybox limitation in Linux dhclient-script

Busybox's implementation of chown and chmod doesn't provide a
"--reference" option used in the latest version of dhclient-script.
This change works around that limitation by using stat to read
ownership and permissions flags and simple chown/chmod calls
supported in both coreutils and busybox.

Patch submitted upstream to ISC, tracked as bug 48771.

Signed-off-by: Haris Okanovic <haris.okanovic@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>