net: disallow to specify multicast MAC address

[Assigning a multicast MAC address to a NIC leads to confusing behavior.
Reject multicast MAC addresses so users are alerted to their error
straight away.

The "net/eth.h" in6_addr rename prevents a name collision with
<netinet/in.h> on Linux.
-- Stefan]

Signed-off-by: Dmitry V. Krivenok <krivenok.dmitry@gmail.com>
Reviewed-by: Amos Kong <kongjianjun@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
