Add basic IP set support

Still todo: pre-populating the sets, documentation
Add ipset support to fina6

While ipsets don't strictly care about v4 vs. v6, it's good pratcice to
make them separately configurable -- if the user chooses to reuse sets,
they can aleays reuse a name. Trying to create a set is idempotent if we
use the right flag (-exist).
Add ipset support to fina6

While ipsets don't strictly care about v4 vs. v6, it's good pratcice to
make them separately configurable -- if the user chooses to reuse sets,
they can aleays reuse a name. Trying to create a set is idempotent if we
use the right flag (-exist).
Actually create v6 ipsets
Merge
Make ipset creation file-based

This is incompatible with the earlier changes, but with good reason: some
people may want to pre-populate IP sets.

TODO: documentation