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 changes has been done in cmp_meta() and its own cmp function
as Florian pointed out.

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

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

nfct_filter_dump_set_attr() will set the bit.

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