libxtables: extend xlate infrastructure

libxtables: extend xlate infrastructure

This infrastructure extends the existing xlate infrastructure:

- Extensions can define set dependencies through .xlate. The resulting
  set definition can be obtained through xt_xlate_set_get().
- Add xl_xlate_set_family() and xl_xlate_get_family() to store/fetch
  the family.

The first client of this new xlate API is the connlimit extension,
which is added in a follow up patch.

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