dnsdist: Refactor 'cannot be used at runtime' handling

[rgacogne]

Short description

I did not manage to avoid repeating the function name, unfortunately, without writing something terribly ugly (mixing function-like macros with lambdas), so for now this will have to do.
Closes #11894.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[chbruyand]

lgtm!