Clean up the 'data' vs 'body' pointers in struct Packet.

Clean up the 'data' vs 'body' pointers in struct Packet.

There's always been some confusion over exactly what it all means. I
haven't cleaned it up to the point of complete sensibleness, but I've
got it to a point where I can at least understand and document the
remaining non-sensibleness.

git-svn-id: http://svn.tartarus.org/sgt/putty@10070 cda61777-01e9-0310-a592-d414129be87e