tun: Dump tun files and tun links

The major issue with dump is -- some info id get via netlink,
some via sysfs and some (!) via opened and attached tun file.
But the latter cannot be created, if there's another one attached
(or the mq device is full with threads).

Thus we have to dump this info via existing tun file and keep one
in memory till the link dump code takes place.

Opposite situation is also possible -- we can have a persistent
unattached device. In this case we have to attach to it, dump
things and detach back.

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