conntrack: labels: remove dead code

unsigned, < 0 is always false.

Signed-off-by: Florian Westphal <fw@strlen.de>
conntrack: introduce clear and equal functions for bitmask object

This patch adds two functions, useful for ulogd IPFIX
output module.

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

This patch adds mark filter for event listener, using same struct
nfct_filter_dump_mark.

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