Allow link-local ipv6 gateway addresses

This is a common pattern used to simplify auto-configuration
of IPv6 Networks by using link-local addresses (such as `fe80::1`)
for any subnet.

Since any v6-enabled interface gets an ip in the `fe80::/64` range,
ips needs to be scoped with the interface (`%$devname`, eg. `%eth0`).

For reference, some places where this is used:

https://wiki.hetzner.de/index.php/Netzkonfiguration_CentOS/en
http://openbsd-archive.7691.n7.nabble.com/IPv6-gateway-fe80-1-needs-a-ping-to-work-td264877.html
http://www.spinics.net/lists/netdev/msg365398.html