network: Adds layer 2 (ARP/NDP) proxy mode

network: Adds layer 2 (ARP/NDP) proxy mode

Adds the lxc.net.[i].l2proxy flag that can be either 0 or 1.

Defaults to 0.

This, when used with lxc.net.[i].link, will add IP neighbour proxy entries on the linked device
for any IPv4 and IPv6 addresses on the container's network device.

Additionally, for IPv6 addresses it will check the following sysctl values and fail with an error if not set:

	net.ipv6.conf.[link].proxy_ndp=1
	net.ipv6.conf.[link].forwarding=1

Signed-off-by: tomponline <thomas.parrott@canonical.com>