[CRIU] c/r of macvlan interfaces

[Tycho Andersen]

Hi guys,

Here's a set that enables checkpoint restore of macvlan interfaces. It seems
like we're getting to the point where we might need some more well designed way
to list the kinds of network devices we can c/r (similar to the `struct fstype`
model we have for filesystems), but I didn't do that as part of this patchset
to minimize the diffs. I'd be happy to do that as part of this work if we think
it is necessary, though.

Tycho