[master] fcb04d9 Add some more "Changes for Developers" to "Upgrading to 6.0".

[Dridi Boukelmoune]

> +  * VMOD authors can use the ``VRT_VSC_*()`` series of functions and
> +    the new ``vsctool`` to create statistics for a VMOD that will be
> +    displayed by varnishstat.  Varnish uses the same technique to
> +    create its counters, so you can look to the core code to see how
> +    it's done.

I forgot to add VSC support to varnish.m4 to streamline its use out of
tree. Since I had such a patch for 5.2 it may only be a case of
cherry-picking it with s/5.2/6.0/ because I tested it back then but it
was refused "until we decide to allow VMODs to do that". I will look
at this tomorrow morning and refrain from checking in the patch if
anything looks suspicious. Since there hasn't been any changes in
varnish.m4 in the last 6 months there should also be no conflict, I'll
consider a conflict suspicious and thus hold back for inclusion.

(at the time I submitted macros for VUTs and VSCs and only the
VUT macro got merged)

We should be fine with vsctool.py, I made sure it would land in the
-dev package for both Debs and RPMs.

Dridi