start: dup std{in,out,err} to pty slave

start: dup std{in,out,err} to pty slave

In the case the container has a console with a valid slave pty file descriptor
we duplicate std{in,out,err} to the slave file descriptor so console logging
works correctly. When the container does not have a valid slave pty file
descriptor for its console and is started daemonized we should dup to
/dev/null.

Closes #1646.

Signed-off-by: Li Feng <lifeng68@huawei.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>