TUN-1615: revert miekg/dns to last known working revision

The last known good commit is 6da3249dfb57fbaa16efafcd8744cee8809d80cd before the first release tag.

vendor/github.com/miekg/dns/dns.go

@@ -55,6 +55,16 @@ func (h *RR_Header) Header() *RR_Header { return h }
 // Just to implement the RR interface.
 func (h *RR_Header) copy() RR { return nil }
 
+func (h *RR_Header) copyHeader() *RR_Header {
+	r := new(RR_Header)
+	r.Name = h.Name
+	r.Rrtype = h.Rrtype
+	r.Class = h.Class
+	r.Ttl = h.Ttl
+	r.Rdlength = h.Rdlength
+	return r
+}
+
 func (h *RR_Header) String() string {
 	var s string