If we don't re-open these after clone, the init process has a pointer to the
parent's /dev/{zero,null}. CRIU seese these and wants to dump the parent's
mount namespace, which is unnecessary. Instead, we should just re-open
stdin/out/err after we do the clone and pivot root, to ensure that we have
pointers to the devcies in init's rootfs instead of the host's.

v2: Only close fds if the container was daemonized. This didn't turn out as
    nicely as described on the list because lxc_start() doesn't actually have
    the struct lxc_container, so it cant see the flag. Instead, we just pass it
    down everywhere.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>