This adds new tunfile_entry::ns_id field and populates
it in standard socket way. Restore uses this ns_id
to choose correct namespace. Note, we could completelly
skip set_netns() on restore in case of !has_ns_id, but
using top_net_ns invents some definite behaviour.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
ktkhai: comment written/code movings
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>