6relayd: Add DHCPv6-PD server support, fix relaying in stateful mode

git-svn-id: svn://svn.openwrt.org/openwrt@36381 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcp6c: Several fixes and improvements
* Add preliminary support for RFC 6603 prefix exclusion
* Fix reconfigure-support and reconfigure authentication
* Set maximum retransmission timeout for solicitations to 2h
* Fix a segfault when parsing IA_PD in advertise messages

git-svn-id: svn://svn.openwrt.org/openwrt@36382 3c298f89-4303-0410-b956-a3cf2f4a3e73
netifd: Improve IPv6-ULA assignment handling

git-svn-id: svn://svn.openwrt.org/openwrt@36383 3c298f89-4303-0410-b956-a3cf2f4a3e73