don't emit ifup hotplug events on dhcp renew - use a new iface hotplug type...

don't emit ifup hotplug events on dhcp renew - use a new iface hotplug type 'update' instead; fixes #3906

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@12671 3c298f89-4303-0410-b956-a3cf2f4a3e73