tun: Restore tun files and tun links

This thing is pretty straightforward -- on netns creation
populate it with tun-s, after this collect tun files, open
and attach them with regular fd-s engine.

One tricky thing -- when populating namespace with tun links
make them all persistent and drop this flag (if required)
later, when the first alive opened appears.

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