lxc-start: fix the container leak when daemonize

When start container with daemon model, we'll have a new daemon
process in lxcapi_start, whose c->numthreads is 2, inherited
from his father. Even his father return to main(), the
lxc_container_put won't affect son's numthreads.

So when daemon stops, he should return to main and do
lxc_container_put again, rather than exit and leave the
container alone.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>