Fix confd procotol design description

The protocol design for confd was missing a description of the fourcc
code which we use to distinguish between different message types, if we
want to completely change the protocol. Adding them so that someone
implementing it can find out.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>