There are other components than the cache process that could benefit
from it, in particular libvcc. The relationship with vas.h is somewhat
unfortunate but that also centralize the Tcheck() logic in pdiff() and
doesn't actually require vas.h unless the macros are used, in which
case it's almost guaranteed that the calling code already included
vas.h in the first place.

The benefits should outweigh the drawbacks.

Conflicts:
	bin/varnishd/cache/cache.h
	bin/varnishtest/vtc_http.c