[netdevice] Add concept of a network device index

[netdevice] Add concept of a network device index

IPv6 link-local socket addresses require some way to specify a local
network device.  We cannot simply use a pointer to the network device,
since a struct sockaddr_in6 may be long-lived and has no way to hold a
reference to the network device.

Using a network device index allows a socket address to cleanly refer
to a network device without worrying about whether or not that device
continues to exist.

Signed-off-by: Michael Brown <mcb30@ipxe.org>