netfilter: nft_compat: add ebtables support

This patch extends nft_compat to support ebtables extensions.

ebtables verdict codes are translated to the ones used by the nf_tables engine,
so we can properly use ebtables target extensions from nft_compat.

This patch extends previous work by Giuseppe Longo <giuseppelng@gmail.com>.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>