[master] 90c4b74 Install cache/cache_varnishd.h as part of $ABI strict

[Dridi Boukelmoune]

> Apologies for my confusion - dist was wrong. Yes, you have moved that header
> from not being installed to being installed. And my impression was that phk
> explicitly intended cache_varnishd to be private to varnishd except for "I
> know what I am doing"-vmods which would use SRCDIR.

Again, if you wish not to rebuild a VMOD when upgrading Varnish except
when a major VRT bump occurs, you are in "I know what I am doing"
territory since the defaults don't do that and you as a VMOD author
have to consider whether your code is VRT-safe.

The $ABI stanza is a major step forward in reliability, I don't want
to move back to the Varnish 3 days of doing things. While it was fine
back then, Varnish 4 made it clear that we could do a lot better and I
believe we have gradually done that. Please read VIP 20 again, there's
a history showing where we come from and how much we iterated since
the VMOD inception.

And we still have room for further improvements, so let's not shoot
our community in the feet and have them go back to pain points of VMOD
building that we managed to erase. For example, the Fedora packaging
policy goes against having more than one source for a given set of
packages, so Ingvar would have to rethink the packaging of varnish and
have it install a source tree somewhere in order to update to Varnish
6, and only then could he finally update the varnish-modules package.

Dridi