Force manual DNAME handling to be acknowledged

Force manual DNAME handling to be acknowledged

Adding proper DNAME support to AsyncDnsServer would add complexity to
its code for little gain: DNAME use in custom system test servers is
limited to crafting responses that attempt to trigger bugs in named.

This fact will not be obvious to AsyncDnsServer users as it
automatically loads all zone files it finds and handles CNAME records
like a normal authoritative DNS server would.

Therefore, to prevent surprises:

  - raise an exception whenever DNAME records are found in any of the
    zone files loaded by AsyncDnsServer,

  - add a new optional argument to the AsyncDnsServer constructor that
    enables suppressing this new behavior, enabling zones with DNAME
    records to be loaded anyway.

This enables response handlers to use the DNAME records present in zone
files in arbitrary ways without complicating the "base" code.

(cherry picked from commit 8a562526f6cdaaab37ce31b20e223537281a3d43)