Submitted the PPTP conntrack/nat helper to the mainline kernel

Following-up some serious testing today, I've finally submitted the latest version of the PPTP helper from the netfilter-2.6.14#pptp tree to the mainline kernel.

With some luck, it will be included before 2.6.14 gets final. It should go in, since it doesn't modify existing code but is merely an addition.

Also, please note that the "ip_conntrack_proto_gre.ko" and "ip_nat_proto_gre.ko" modules are gone with that 3.x version of the PPTP helper. The respective code has been integrated into ip_{conntrack,nat}_pptp.ko. My initial dream of doing some generic (non-PPTP) GRE connection tracking has evaporated, and thus the PPTP helper now really only handles the special case of pptp-GRE.