odhcp6c: various fixes

* Make SOL_MAX_RT configurable and default back to 120s
* Prefer servers with bigger prefixes (allows to use Comcast /60)
* Don't hang indefinitely if DHCPv6 REQUEST is not answered

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38393 3c298f89-4303-0410-b956-a3cf2f4a3e73