checkpoint: add the empty-ns option
checkpoint: add the empty-ns option

For example:
./runc checkpoint --empty-ns network CTID

In this case criu creates a network namespace, but doesn't restore it.

We are going to use this option to restore docker containers and
Docker sets a hook to restore a network namespace.

https://github.com/xemul/criu/issues/165
Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>

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