dhcp: Get rid of ifconfig/route in dhclient-script

Copying: https://lists.isc.org/pipermail/dhcp-users/2011-January/012910.html
    
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    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.
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Signed-off-by: Muhammad Shakeel <muhammad_shakeel@mentor.com>