lxc-ls: Implement support for nested containers

Add initial support for showing and querying nested containers.

This is done through a new --nesting argument to lxc-ls and uses
lxc-attach to go look for sub-containers.

Known limitations include the dependency on setns support for the PID
and NETWORK namespaces and the assumption that LXCPATH for the sub-containers
matches that of the host.

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