criu: add support for external net namespaces

It works like other external resources.
A user specify which namespaces are external and have not to be dumped.
On restore, the user gives file descriptors to preconfigured namespaces.

How to use:
dump:
        --external net[INO]:KEY
restore:
        --inherit-fd fd[NSFD]:KEY

The test script contains more details how to use this:
test/others/netns_ext/run.sh

Acked-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>