ip6: Implement IPv6 address sharing

ip6: Implement IPv6 address sharing

This allows the same IPv6 address to exist on more than one
interface. Whenever dhcpcd address an IPv6 address, it will
advertise it along with the hardware address of the preferred
interface.

This is heavliy reliant on the kernel supporting this as it's the
kernel that handle the Duplicate Address Detection.
In a nutshell it needs to support RFC 7527 and ignore NA packets
from any hardware address the host owns.

Currently the only known kernel that fully supports this is
NetBSD-8.99.27