add RFC4191 support

[goertzenator]

Resolves issue #102.

Implement RFC4191 support for Route Information options in Router Advertisements. This option allows a router to provide routes for a specific prefix instead of only providing default routes.

My use case is that I have a ULA network with a VPN router and no IPv6 internet access. Using this router as the default route does not work well because the host then thinks it can reach the Internet. By instead advertising a route just for the ULA prefix hosts will immediately know they cannot reach the Internet through this router.

I've tested this PR in a VM in this exact configuration and it seems to work quite well. I changed the radvd config on the fly to remove and add the route and it behaved as expected.

[goertzenator]

Thanks for the prompt review, I appreciate it!

[rsmarples]

Thanks for your contribution!