This adds CONFIG_IP_NF_PPTP:
Connection tracking and NAT support for PPTP.  Using this, you can track
PPTP/GRE connections and do SNAT/DNAT.  You have to load the following modules
for connection tracking:
	ip_conntrack_proto_gre
	ip_conntrack_pptp
for NAT:
	ip_nat_proto_gre
	ip_nat_pptp

The GRE connection is marked as RELATED to the TCP session on port 1723, so all
you need is something like

iptables -j ACCEPT -m state --state RELATED,ESTABLISHED
iptables -j ACCEPT -d my_pptp_server -p tcp --dport 1723 -m state --state NEW


Note that this code currently has limitations
- can only NAT connections from PNS to PAC
- doesnt' support multiple calls within one session
- you have to recompile your iptables userspace program since some 
  structure sizes change
