odhcp6c: add support for option passthru

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40824 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcp6c: export passthru options as netifd data

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40825 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcp6c: improve passthru filter

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40826 3c298f89-4303-0410-b956-a3cf2f4a3e73