netfilter: nf_tables: dump sets in all existing families

This patch allows you to dump all sets available in all of
the registered families. This allows you to use NFPROTO_UNSPEC
to dump all existing sets, similarly to other existing table,
chain and rule operations.

This patch is based on original patch from Arturo Borrero
González.

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