odhcp6c: several fixes

* Reduce RA/DHCPv6 spam filter to 30s by default and make it configurable
* Don't set nd_ra_{reachable,retransmit] to 0 when received in RAs

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39775 3c298f89-4303-0410-b956-a3cf2f4a3e73