ns: Introduce ns type

We (may) have 3 types of namespace objects in criu -- criu's one,
root task's one and others. All of them sometimes make sense and
we differentiate them in a weird way -- by checking the ns->pid
field against getpid() or by comparing with root_item's.

The proposal is to mark ns_id objects explicitly with type field.

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