C/R: Network namespace and device support (v2)

This set adds support for checkpointing network namespaces and devices,
with in-kernel restart.

Basic support includes veth pair and loopback adapters and allows me to
checkpoint and restart a containerized copy of sendmail with a veth
bridged to a physical network.  Follow-on work is detailed in the header
of the last patch.

Changes in v2 only to the final patch, detailed there.

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