Netlink: Handle onlink flag on BSD-Netlink

Netlink: Handle onlink flag on BSD-Netlink

On BSD, the onlink flag is not tracked or reported by kernel. We are
using an heuristic that assigns the onlink flag to routes scanned from
the kernel. We should use the same heuristic even in BSD-Netlink
case, as the onlink flag is not reported here too.

Thanks to Björn König for the original patch.