Porting PPTP conntrack/nat helpers to 2.6.x

I've always refused to do the port of the PPTP conntrack/NAT helper I wrote for 2.4.x because there's higher priority items on my agenda.

Apparently it helped, as I was told Mandrake did a port to 2.6.x. I thought that is great news, and I thought it'd take an hour or so to get it merged.

Unfortunately that 'port' was totally incomplete. NAT couldn't have worked at all, and if you sent it a nonlinear TCP packet it would very likely crash your kernel.

In the end I spent the whole afternoon at it, with a resulting patch that is about the same size as the original code :(

The code is now in our subversion repository, I didn't have the time test it so far, so any testing you (yes, you, the reader) might give it would be appreciated.