daemon: fix the wrong pid in daemon model

daemon: fix the wrong pid in daemon model

When you start a container in daemon model, you have at least
3 processes:
 1. The command the user start (lxc-start -d)
 2. The backgrounded fork of that command after start() is done
 3. The container init process

In PID file, we need (2), but currently we are writing (1),
this is wrong because (1) exits as soon as the container is
started, it's complately useless.

So we write pid after daemonize, so that we'll always write
the right pid to PID file.

Reported-by: Stephane Graber <stgraber@ubuntu.com>
Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>