9p: add new protocol support code

This adds a new protocol processing support code based on Anthony Liguori's
9p library code.  This code performs protocol marshalling/unmarshalling using
printf like strings to represent protocol elements.  It is my intent to use
them to replace the current functions in conv.c as well as the 
p9_create_* functions.

This should make the client implementation much more clear, and also make it
much easier to add new protocol extensions by limiting the number of places
in which changes need to be made.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>