This removes the need for non-VRT VMODs to require a Varnish source tree
to build, especially since varnish.m4 only supports Varnish installations
today and would leave VMOD authors on their own in order to build from
a source tree. Since $ABI defaults to strict, any VMOD author actively
choosing vrt compliance is responsible for ensuring it only uses blessed
symbols.

In our official packages, they land in the varnish-dev deb and the
varnish-devel RPM:

    $ rpm --query --package --list varnish-trunk-1.el7.x86_64.rpm |
    > grep cache_varnishd.h

    $ rpm --query --package --list varnish-devel-trunk-1.el7.x86_64.rpm |
    > grep cache_varnishd.h
    /usr/include/varnish/cache/cache_varnishd.h

You will have to trust me for the Debs as it's a bit cumbersome for me to
build them locally.

Not that I'm planning another stab, but please review and let me know if
anything needs clarification.

Currently not backends.

This commit comes with the realization that finding a workaround
for #1799 is even harder than I thought.

Unfortunately we cannot close issue #1799 for the reasons described
in the documentation.

Thanks to @nigoroll for valuable feedback.

To be taken up again after 6.0.

Thank you to phk for spotting this!

Fixes: #2607

Vaild values are 'auto' or 'manual', default 'auto'. With 'auto' you
get the auto-generated SYNOPSIS in the VMOD docs. With 'manual', the
SYNOPSIS is just skipped, so the VMOD author has to write it.