network: Unbind DHCPv6 from underlying interfaces

This allows IPv6 to set up without IPv4 being up thus
IPv6-only or IPv6+DS-Lite working with the default config.

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42848 3c298f89-4303-0410-b956-a3cf2f4a3e73