Skip the argument names when writing the prototypes
Polish

Improve consistency, add argument names and rewording.
Cosmetic
document vcl_synth
typo
Make WS_Alloc() return a void* like any sane memory allocator should
Allocate bo->htc from the bo->ws when we needed it.
move vbc from bo to bo->htc
Update forgotten varnishlog example to 4.0 syntax.

Fixes: #1610
Remove description lint.
Improve counter descriptions.

Make the help text for the VSC counters a bit more explanatory.
Unbreak backend retries
whitespace ocd before further editing
move the label to the right place
generate svgs
get vcl_purge via vcl_hash right
add svg versions of the graphviz-generated graphs to html docs
complete overhaul of vcl-built-in-subs.rst documentation

Add missing bits and sync with reality. Relevant fixes:

* a synth object is never cached
* by a backend_response object may be
fix out-of-tree make dist
remote debugging of jenkins build failures via git pushes