Resolve "apex in name_external may be invalid when using dual stack servers" [v9_18]

When using dual-stack-servers the covering namespace to check whether answers are in scope or not should be fctx->domain. To do this we need to be able to distingish forwarding due to forwarders clauses and dual-stack-servers. A new flag FCTX_ADDRINFO_DUALSTACK has been added to signal this.

(cherry picked from commit dfbffd77)

Closes #3607 (closed)