the directory where the .vtc file lives.

Suggested by:	HAproxy

Without it, graphing Timestamp tags was impossible for all practical purposes

Related to #2626.

We already check the -a user input and emit an error if the path is
too long.

bprintf() should help persuade static analysis tools that we won't
overflow the fixed-size buffer.

We currently suppress the error message that actually explains the
problem.

Ref: #2610 with another thank you to @fgsch

Ref: #2610

Thank you to @fgsch for spotting

Apologies to everyone for my insufficient understanding of the agreement
about which fixes we push during the soak in period after release.

Fixes #2610

Initially written and tested for Varnish 5.2, but not included then
because VMODs weren't allowed custom counters yet. Tested again with
success for Varnish 6.0 and documented.

we are not even using it in our own code yet, so I feel uneasy
about pointing devs to them.

We do not list bugs that were both introduced and fixed between the two versions.

And remove the last XXX from the docs.