I noticed that some shells like ye olde bash available on SmartOS don't
treat the following construct as single a token:

    ${dir}/${file}

The following expands a single token on the RHS even if the variable
contains spaces:

    var=${dir}

But a token involving a regular string too needs to be protected by
double quotes:

    var="${dir}/${file}"

Even when none of the variables contain blanks.

Refs #2139

IOW if we ended up in vcl_backend_error{} or failed after returning
delivery in vcl_backend_response{} (e.g. we did not receive enough bytes)
we will bump this counter.

it slows things down.

Many of these could benefit from a rethink now that we can expect
a pattern also on shell commands which succeed.

Test varnishd in -F mode with out VCL

be found are options to "shell".  Deprecate err_shell.

Structure output to always be prefixed by "shell" since we're running
at the top level.

Also prefix commands with "exec 2>&1 ;" to join stdout+stderr by default.
This still doesn't cover the shells stderr in case of bad shell syntax,
which is annoying, but not worth fighting for right now.

to start the child process.

Add bad_backend macro and use it instead of ${bad_ip}/port,
should shave a few seconds here and there.
While here also replace some unneeded ${bad_ip} entries.

If you give multiple -f arguments, the resulting VCL's will be
"boot0", "boot1", ... "boot668", "boot"

And the last one will be the active one.

The plan is to also allow both specification of an (optional) label and
which VCL is to be the active one, but I want to think about if we should
shoehorn that into -f or use a different option letter.

all other modes than -d.

Allow -f '' as an override for people who really want this.

Fixes: #2167

Fixes #1704

processed until after we switched VCL.

Fixes #2052

Fixes #2078

general cleanup and polishing.