Apply cache TTL policy to DNAME and synthesized CNAME on wire path (#1418)

Apply cache TTL policy to DNAME and synthesized CNAME on wire path (#1418)

When the scrubber synthesizes a CNAME from a DNAME (authority omits CNAME),
apply cache-min-ttl/cache-max-ttl to both DNAME and synthesized CNAME in
msg_parse so they stay equal and respect config (RFC 6672).

- iterator/iter_scrub.c: In synth_cname_rrset(), clamp TTL to [MIN_TTL,
  MAX_TTL] when !SERVE_ORIGINAL_TTL and write back to both synth CNAME
  and DNAME rrset. Removes FIXME.