Since we killed lxc-monitord we rely on the container's command socket to wait
for the container. This doesn't work nicely on daemonized startup since a
container's init process might be something that is so short-lived that we
won't even be able to add a state client before the mainloop closes. But the
container might still have been RUNNING and executed the init binary correctly.
In this case we would erroneously report that the container failed to start
when it actually started just fine.
This commit ensures that we really all cases where the container successfully
ran by switching to a short-lived per-container anonymous unix socket pair that
uses credentials to pass container states around. It is immediately closed once
the container has started successfully.
This should also make daemonized container start way more robust since we don't
rely on the command socket handler to be running.

For the experienced developer: Yes, I did think about utilizing the command
socket directly for this. The problem is that when the mainloop starts it may
end up end accept()ing the connection that we want
do_wait_on_daemonized_start() to accept() so this won't work and might cause us
to hang indefinitely. The same problem arises when the container fails to start
before the mainloop is created. In this case we would hang indefinitely as
well.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>