Ref: 90c4b74 discussion

	#varnish-hacking log:

(09:53:32) slink: dridi phk so is it really agreed policy that indeed we
return to installing all headers which $ABI strict vmods do/could
require?
(09:54:48) slink: and if yes, what is the purpose of having
cache_varnishd.h over only cache.h then?
(09:58:54) phk: slink, that was not the outcome I anticipated, but if
that is how it will work, then that is how it will work.
(09:59:26) phk: slink, the purpose is to allow people to write VMODs
which doesn't require recompilation everything