Add a new macro IF_LLSOCKADDR which maps a ifnet pointer to the

Add a new macro IF_LLSOCKADDR which maps a ifnet pointer to the
associated link layer sockaddr. Use it to implement IF_LLADDR.
Replace most references of ifnet_addrs with one of those macros.

This makes the sometimes explicitly encoded assumption of each
interface having a link-layer address implicit and is the
preparation for removing ifnet_addrs.