src: use cache infrastructure for set objects

src: use cache infrastructure for set objects

This patch populates the cache only once through netlink_list_sets() during
evaluation. As a result, there is a single call to netlink_list_sets().

After this change, we can rid of get_set(). This function was fine by the time
we had no transaction support, but this doesn't work for set objects that are
declared in this batch, so inquiring the kernel doesn't help since they are not
yet available.

As a result from this update, the monitor code gets simplified quite a lot
since it can rely of the set cache. Moreover, we can now validate that the
table and set exists from evaluation path.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>