Fix unprivileged networking

If we are unprivileged and have asked for a veth device, then create
a pipe over which to pass the veth names.

Network-related todos:
1. set mtu on the container side of veth device
2. set mtu in lxc-user-nic.  Note that this probably requires an
   update to the /etc/lxc/lxc-usernet file :(

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>