odhcp6c: minor bug fixes

git-svn-id: svn://svn.openwrt.org/openwrt@36123 3c298f89-4303-0410-b956-a3cf2f4a3e73
feeds.conf.default: switch LuCI feed to nbd.name git mirror

git-svn-id: svn://svn.openwrt.org/openwrt@36124 3c298f89-4303-0410-b956-a3cf2f4a3e73
iptables: don't use --enable-ipv6 if IPv6 is disabled

git-svn-id: svn://svn.openwrt.org/openwrt@36125 3c298f89-4303-0410-b956-a3cf2f4a3e73