[NETFILTER] nfnetlink_log: add sequence numbers for log events

By using a sequence number for every logged netfilter event, we can
determine from userspace whether logging information was lots somewhere
downstream.

The user has a choice of either having per-instance local sequence
counters, or using a global sequence counter, or both.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>