Add new NFLOG target to allow use of nfnetlink_log for both IPv4 and IPv6.
Currently we have two (unsupported by userspace) hacks in the LOG and ULOG
targets to optionally call to the nflog API. They lack a few features,
namely the IPv4 and IPv6 LOG targets can not specify a number of arguments
related to nfnetlink_log, while the ULOG target is only available for IPv4.
Remove those hacks and add a clean way to use nfnetlink_log.

Signed-off-by: Patrick McHardy <kaber@trash.net>