Add debug.Hexdump (#1902)

Allow plugins to dump messages in text pcap to the log. The forward
plugin does this when a reply does not much the query.

If the debug plugin isn't loaded Hexdump and Hexdumpf are noop.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/forward.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/debug"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -131,6 +132,8 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 		// Check if the reply is correct; if not return FormErr.
 		if !state.Match(ret) {
+			debug.Hexdumpf(ret, "Wrong reply for id: %d, %s/%d", state.QName(), state.QType())
+
 			formerr := state.ErrorMessage(dns.RcodeFormatError)
 			w.WriteMsg(formerr)
 			return 0, nil