netfilter: ebtables: Add string filter

This patch is part of a proposal to add a string filter to
ebtables, which would be similar to the string filter in
iptables. Like iptables, the ebtables filter uses the xt_string
module.

Signed-off-by: Bernie Harris <bernie.harris@alliedtelesis.co.nz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>