[NETFILTER]: x_tables: add NFLOG target

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>
diff --git a/include/linux/netfilter_ipv6/ip6t_LOG.h b/include/linux/netfilter_ipv6/ip6t_LOG.h
index 060c1a1..0d0119b 100644
--- a/include/linux/netfilter_ipv6/ip6t_LOG.h
+++ b/include/linux/netfilter_ipv6/ip6t_LOG.h
@@ -6,7 +6,7 @@
 #define IP6T_LOG_TCPOPT		0x02	/* Log TCP options */
 #define IP6T_LOG_IPOPT		0x04	/* Log IP options */
 #define IP6T_LOG_UID		0x08	/* Log UID owning local socket */
-#define IP6T_LOG_NFLOG		0x10	/* Log using nf_log backend */
+#define IP6T_LOG_NFLOG		0x10	/* Unsupported, don't use */
 #define IP6T_LOG_MASK		0x1f
 
 struct ip6t_log_info {