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.
Also, we should become session leader.

Closes #1646.
Closes #1951.
Signed-off-by: Li Feng <lifeng68@huawei.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>