net: Add ability to dump external links with plugins

If we meet a link we cannot dump we call plugin to check
whether it's the link, that should be treated as external.

Note, that on restore we don't call any plugins, but
consider the setup-namespace script to move the respective
link into the namespace. Links are not hierarchical and
can be moved between namespaces easily, so it's OK to
delegate the link creation to the script.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>