It turns out declaring typedefs in coccinelle works better with the
typedef keyword. Reusing the same keyword for the same purposes seems
so obvious in hindsight.

What happened before was that a generic type would be created, making
VCL_BOOL or VCL_VOID match virtually any type, completely defeating
the isomorphism purpose. With this out of the way, we can actually
resume the effort of adding more typedefs.

To summarize:

- we should declare a typedef as an isomorphism
- we should define macros as such

I'm considering renaming tools/coccinelle/vdef.h to varnish.h to match
the varnish.iso file name and wrapping the README instructions in a new
shell script.