netns: Introduce a routine that runs ip tool for dump/restore

The thing is that the ip utililty is very likely to have support
for ifaddrs and routes dumping and restore (rtnl messages are
symmetrical wrt dump/restore and this can be easily dump in there).

Thus, the best we can do it just use this tool for that and carry
the "raw" images with ifaddrs and routes dump.

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