odhcp6c: several bugfixes and improvements

* Fixed broken stateless-mode (fixes #14863)
* Support for multiple IAIDs, improved server compatibility,
  improved NTP/SNTP support and other fixes (thanks to T-Labs)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39433 3c298f89-4303-0410-b956-a3cf2f4a3e73