odhcp6c: avoid busylooping with strangely behaving dhcpv6 servers

Thanks to Dave Taht for debugging and thanks to Comcast for
shipping strangely behaving software so I can fix some corner cases.

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43415 3c298f89-4303-0410-b956-a3cf2f4a3e73