conntrack: api: add two new bitmask functions

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

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
Signed-off-by: Florian Westphal <fw@strlen.de>
qa: add tests for new bitmask functions

for nfct_bitmask_clear() and nfct_bitmask_equal()

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
Signed-off-by: Florian Westphal <fw@strlen.de>