odhcp6c: Multiple improvements and fixes

* Various RFC compliance fixes (thanks to Hans Dedecker)
* Fix RA spam filter logic causing ipv6 connection issues
* Fix parameters -F and -P being order dependent

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38577 3c298f89-4303-0410-b956-a3cf2f4a3e73