Get rid of packet data members of header structures.

Instead, have a special macro using the size of the header structure to
find the offset of the packet data.

This means that:

	1) you don't have to throw "-{size of data member}" into the
	   macros that calculate the sizes of the header structures;

	2) you don't have a bunch of randomly-chosen data field sizes;

	3) you don't have sizes of 0, which cause problems with
	   compilers that don't support zero-length arrays;

	4) you don't have some apparently-incorrect "-{size of data
	   member}" values (if they're correct, please fix the structure
	   definitions).

Change-Id: Iea368b83fa2d184bd6df453d51756e4749714e2c
Reviewed-on: https://code.wireshark.org/review/6082
Reviewed-by: Guy Harris <guy@alum.mit.edu>