lxc: make container's init process session leader

lxc: make container's init process session leader

This patch forces container's init process, to become a session leader,
that is its session ID is made the same as its process ID.
That might seem unnecessary in general, but if we want to checkpoint a
container with CRIU, which is needed for container migration,
we must ensure that the SID of each process inside the container points
to a process that lives in the same PID namespace as the container.
Therefore, we force that the session leader is the init.

Signed-off-by: Katerina Koukiou <k.koukiou@gmail.com>