Use --root instead of --aufs-root

When dumping Docker containers using the AUFS graph driver, we can
use the --root option instead of --aufs-root for specifying the
container's root.  This patch obviates the need for --aufs-root
and makes dump CLI more consistent with restore CLI.

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
zdtm: fallback to mknod(/dev/ptmx) when CONFIG_DEVPTS_MULTIPLE_INSTANCES is not set

A kernel without that option configured does not have /dev/pts/ptmx, so
fallback to the previous way of creating it using mknod instead.

The previous code was trying to bind mount ptmx on top of a symlink, which does
not actually work... Keep only the symlink call and use a relative symlink
instead. Adjust the error message of the symlink case to mention symlink()
instead of mknod() and also /dev/ptmx instead of /dev/pts.

Tested:
- zdtm test suite runs on ^ns/static/.* before and after the change.
- Same on a kernel with CONFIG_DEVPTS_MULTIPLE_INSTANCES unset.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
restore: die if init fails in --restore-detached mode

When in --restore-detached (i.e. root_as_sibling) mode, we ptrace(PTRACE_SEIZE)
the root task to receive its SIGCHLD in case one of its child tasks dies.
However, we don't receive a SIGCHLD if the root task itself dies, so we must
explicitly abort.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
restore: don't restore cg props if task restore fails

Once the task restore has failed, we can just abort, no need to restore the cg
props.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>