Forbid duplicated namespaces with same type.

[WeiZhang555]

Related runc PR: opencontainers/runc#1150

Signed-off-by: Zhang Wei [email protected]

[wking]

Instead of using an array for a structure where order is insignificant
and duplicates are not allowed, I'd rather use an object (since the
just-landed #584, all of our objects forbid duplicates and do not have
significant ordering). I've filed #598 with that approach. I
understand that migrating from an array to an object will be painful,
but I think it's worth it if we have no reason to use arrays for
portable ordering or duplication. One benefit of using an object is
that we get much easier add-or-replace and removal code than hacks
like this 1.

[crosbymichael]

LGTM

[hqhq]

LGTM