Changes

- External sharing is not supported (converted to internal sharing after c/r) as reasonable container environments should not allow it due to security reasons, and implementing it's lookup would lead to bad performance (host mountinfo reading).

- External sharing is not supported (converted to internal sharing after c/r) as reasonable container environments should not allow it due to security reasons, and implementing it's lookup would lead to bad performance (host mountinfo reading).

- External slavery is supported for mountpoint external mounts and the root mount. It is detected when criu can't lookup master_id of the mount across shared_ids in container mount namespaces. CRIU relies that mountpoint external source provides right shared/slave mount to copy sharing from. Everything else is considered as internal sharing/slavery.

- External slavery is supported for mountpoint external mounts and the root mount. It is detected when criu can't lookup master_id of the mount across shared_ids in container mount namespaces. CRIU relies that mountpoint external source provides right shared/slave mount to copy sharing from. Everything else is considered as internal sharing/slavery.