odhcp6c: various bugfixes
* Don't prematurely send RENEW / REBIND when T1 or T2 is > 1 day
* Add "norelease" option to not send a RELEASE when stopping

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35902 3c298f89-4303-0410-b956-a3cf2f4a3e73