Fixes #2084

To avoid another regression, a test case now passes a probe to the debug
VMOD. This problem was discovered while working on #2068.

Also try to clarify CLIS_PARAM (= nonsensical parameter) from
CLIS_CANT (= params make sense, but you can't do that.)

Spotted by: Dridi Boukelmoune

Thanks @nigoroll for the suggestion.

This allows to test vgc_inistep only once and make it easier to work for
the compiler.

change resp.msg to resp.reason

Spotted by: Nils Goroll

E.g. set req.http.foo = req.http.bar ~ "foobar"

Related to #2075.

This reverts commit 7e3e34a4251536919c51648a7354d521c4e002b7.

Previously, std.log had printf-semantics for the case that there is no ctx->vsl.

Now, for all cases, it just prints the given STRING_LIST

Fixes the example given in #2063

This introduces a way to check whether a director returned no backend
and act upon it.

Closes #2060