odhcpd: multiple fixes

* Rewrite ndp proxy using kernel proxying
* Aid flash-renumbering in hybrid DHCPv6-mode
* Unicast RAs to RS senders
* Add support for router address

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42944 3c298f89-4303-0410-b956-a3cf2f4a3e73