[NETFILTER]: Add new "nfnetlink_log" userspace packet logging facility

This is a generic (layer3 independent) version of what ipt_ULOG is already
doing for IPv4 today.  ipt_ULOG, ebt_ulog and finally also ip[6]t_LOG will
be deprecated by this mechanism in the long term.

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