net: ipv4: Re-join multicast groups if needed

If network interface goes operative down, then re-join user defined
IPv4 multicast groups that where joined earlier when the interface comes
up again. If user takes the network interface down manually (admin down),
then the user specific groups are not re-joined. This works now the same
way as IPv6 re-joining.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>