odhcpd: Fix RA interval + add support for prefixes > 64 and < 97

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39604 3c298f89-4303-0410-b956-a3cf2f4a3e73
odhcp6c: send reconfigure-accept again to be compliant

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39605 3c298f89-4303-0410-b956-a3cf2f4a3e73