Rename Netif to Client

Signed-off-by: David Scott <dave.scott@citrix.com>
Satisfy V1.NETWORK by moving types within the functor

Signed-off-by: David Scott <dave.scott@citrix.com>
Rename the packed module 'Netchannel'

So now mirage-net-xen (should also be renamed) has:

Netchannel.Client.Make(MEMORY)(EVENTS)(CONFIGURATION)
 : V1_LWT.NETWORK

Signed-off-by: David Scott <dave.scott@citrix.com>