[NETFILTER]: Add SIP connection tracking helper

Add SIP connection tracking helper. Originally written by
Christian Hentschel <chentschel@arnet.com.ar>, some cleanup, minor
fixes and bidirectional SIP support added by myself.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/linux/netfilter_ipv4/ip_conntrack_sip.h
Simple merge

diff --cc net/ipv4/netfilter/Kconfig
Simple merge

diff --cc net/ipv4/netfilter/Makefile
Simple merge

diff --cc net/ipv4/netfilter/ip_conntrack_sip.c
Simple merge

diff --cc net/ipv4/netfilter/ip_nat_sip.c
Simple merge