netns: dump and restore network namespace ID-s

In each network namespace we can set an id for another network namespace
to be able to address it in netlink messages.

For example, we can say that a peer of a veth devices has to be created
in a network namespace with a specified id. If we request information about
a veth device, a kernel will report where a peer device lives.

An user are able to set this ID-s, so we have to dump and restore them.

v2: add more commetns
v3: make a union of nsfd_id and ns_fd, they are not used together
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>