restore: add mount id-s in the ns_ids list (v4)

Currently ns_ids list is filled only on dump. Soon we'll need this
list for mount namespaces on restore, e.g. to know which tasks share
the namespaces.

v2: merge the patch "namespace: add a function to search an ns_id
item by id" into this one.
v3: add prefix rst_ to add_ns_id
v4: look up namespace by two values -- type AND ID

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>