moving sockbuf in to its own header

[Kip Macy]

TOE drivers  need to be able to directly enqueue data in to a socket
buffer and thus benefit from having knowledge of sockbuf internals.
However, there is no need for them to know about other socket
definitions. Thus I would like to move sockbuf and accompanying
definitions to their own header.

This is a fairly straightforward change so I don't really see the need
to wait more than a few days for feedback:

http://www.fsmware.com/sockbuf.h.diff