mbuf: add named points inside the structure

mbuf: add named points inside the structure

Add markers or "labels" at given points inside the mbuf which can be
used instead of individual fields to identify the start of logical
sections inside the mbuf.

The use of typedefs and dummy fields was chosen over using unions
because of a couple reasons:
* unions cause an extra level of indentation (more likely two levels as
  a union containing a struct for multiple fields would be needed). This
  makes the lines longer than they need to be and increases the need for
  wrapping. [This was the main reason]
* with markers, you can apply multiple markers at the same point if
  wanted.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>