dhcp: do not use ifconfig/route in dhclient-script

dhcp: do not use ifconfig/route in dhclient-script

The original patch was submitted on ISC's mailing list but it didn't get merged
for some reason, it can be found here;

https://lists.isc.org/pipermail/dhcp-users/2011-January/012910.html

"The ISC DHCP's dhclient-script for Linux still uses the legacy commands
'ifconfig' & 'route' to manage IPv4 network addresses & routes,
while it uses iproute2's new "ip" command to manage IPv6 network data.

This requires users & distributions to have both types of commands installed

The attached patch tries to improve the situation by replacing the
old "ifconfig" resp. "route" commands with their "ip" counterparts.

It even improves the logic a bit by only flushing the IPv4 addresses
instead of taking down the whole interface, which may still have active IPv6
connections."

Signed-off-by: Fahad Usman <fahad.usman@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>