resolved: fix NSEC proofs for missing TLDs

resolved: fix NSEC proofs for missing TLDs

For the wildcard NSEC check we need to generate an "asterisk" domain, by
prepend the common ancestor with "*.". So far we did that with a simple
strappenda() which is fine for most domains, but doesn't work if the
common ancestor is the root domain as we usually write that as "." in
normalized form, and "*." joined with "." is "*.." and not "*." as it
should be.

Hence, use the clean way out, let's just use dns_name_concat() which
only exists precisely for this reason, to properly concatenate labels.

There's a good chance this actually fixes #5029, as this NSEC proof is
triggered by lookups in the TLD "example", which doesn't exist in the
Internet.