nft cache updates

[Patrick McHardy <kaber@trash.net>]

Hi Pablo,

I'm wondering what the rational for the current cache update behaviour is.
The changelog states it is somehow related to the requested command, but
that doesn't seem to be true.

Even "nft describe" fails with EPERM as user since the cache appears to be
initialized unconditionally, which is a bit unfortunate. Also I used to
test things parsing, evaluation and even netlink generation without actually
adding those rules as user, which does not work anymore. This might be harder
to get working again, but I'm not sure why we do a full initialization anyways.
The only thing that appears to be needed are sets, and those only in some
specific circumstances.

Cheers,
Patrick