Expose underlying close_all_fds config value via API

Being able to set close_all_fds via API would be usefull for the
situations like running an application (let's say web server)
that controls the lifecycle of the container using the LXC API.
We don't want forked process to inherit parent's resource (file, socket, ...)

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>