netfilter: nf_tables: cleanup nf_tables.h

The transaction related definitions are squeezed in between the rule
and expression definitions, which are closely related and should be
next to each other. The transaction definitions actually don't belong
into that file at all since it defines the global objects and API and
transactions are internal to nf_tables_api, but for now simply move
them to a seperate section.

Similar, the chain types are in between a set of registration functions,
they belong to the chain section.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>