netfilter/ct: support optional CTA_ZONE attribute

netfilter/ct: support optional CTA_ZONE attribute

Newer kernels support conntrack zones, which help to partition the
conntrack table into virtual conntrack tables.

This patch is for adding support for the optional attribute, adds
setters and getters, and adds support for the zone ID in the conntrack
dumper.

An example entry in NL_DUMP_LINE format looks like:

  tcp SYN_SENT 10.128.128.99:43354 <-> 10.128.129.20:22 zone 1

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Thomas Graf <tgraf@suug.ch>