iptables: xtables-eb: fix renaming of chains

Renaming of chains is not working. and ebtables-compat gets:
 libnftnl: attribute 0 assertion failed in chain.c:159

This patch brings back the parser code of the original ebtables tool:
 http://git.netfilter.org/ebtables.old-history/tree/userspace/ebtables2/ebtables.c#n652

I adapted the original parser code to fit in the new environment. Also tried to
keep original error messages as much as possible.

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