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

you need to explicitly enable (params.set feature +http2)

major lurking.

White-space polishing and fix extraneous 'd' appended to expect
values in previous commit.

This fixes a regression against 4.1

Reported by Dridi