extensions: add ebt 802_3 extension

This patch adds the first ebtables extension to ebtables-compat.
The original 802_3 code is adapted to the xtables environment.

I tried to mimic as much as possible the original ebtables code paths.

With this patch, ebtables-compat is able to send the 802_3 match to the kernel,
but the kernel-to-userspace path is not tested and should be adjusted
in follow-up patches.

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

With this patch:

% sudo ebtables-compat -L --Lc
Bridge table: filter

Bridge chain: INPUT, entries: 0, policy: ACCEPT
-j ACCEPT , pcnt = 123 -- bcnt = 123

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