conntrack: compare mark and zone as special

This patch enables comparison of 0 value with mark and zone since
both CTA_MARK and CTA_ZONE are not set in case of its value is 0.
These comparison has changed in cmp_meta() as Florian pointed out.

This enables `conntrack -L --zone 0' to work as expctedly too.

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
conntrack: remove duplicate code

This patch removes duplicate code from filter_dump.c

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
conntrack: add userspace dump filter

This patch adds more type to filter_dump, working in userspace.

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
conntrack: examples

temporary commit