netfilter: ctnetlink: dump entries from the dying and unconfirmed lists

This patch adds a new operation to dump the content of the dying and
unconfirmed lists.

Under some situations, the global conntrack counter can be inconsistent
with the number of entries that we can dump from the conntrack table.
The way to resolve this is to allow dumping the content of the unconfirmed
and dying lists, so far it was not possible to look at its content.

This provides some extra instrumentation to resolve problematic situations
in which anyone suspects memory leaks.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>