Cleanup: put middleware helper functions in pkgs (#245)

Move all (almost all) Go files in middleware into their
own packages. This makes for better naming and discoverability.

Lot of changes elsewhere to make this change.

The middleware.State was renamed to request.Request which is better,
but still does not cover all use-cases. It was also moved out middleware
because it is used by `dnsserver` as well.

A pkg/dnsutil packages was added for shared, handy, dns util functions.

All normalize functions are now put in normalize.go

middleware/pkg/response/classify.go

@@ -0,0 +1,52 @@
+package response
+
+import "github.com/miekg/dns"
+
+type Type int
+
+const (
+	Success    Type = iota
+	NameError       // NXDOMAIN in header, SOA in auth.
+	NoData          // NOERROR in header, SOA in auth.
+	Delegation      // NOERROR in header, NS in auth, optionally fluff in additional (not checked).
+	OtherError      // Don't cache these.
+)
+
+// Classify classifies a message, it returns the Type.
+func Classify(m *dns.Msg) (Type, *dns.OPT) {
+	opt := m.IsEdns0()
+
+	if len(m.Answer) > 0 && m.Rcode == dns.RcodeSuccess {
+		return Success, opt
+	}
+
+	soa := false
+	ns := 0
+	for _, r := range m.Ns {
+		if r.Header().Rrtype == dns.TypeSOA {
+			soa = true
+			continue
+		}
+		if r.Header().Rrtype == dns.TypeNS {
+			ns++
+		}
+	}
+
+	// Check length of different sections, and drop stuff that is just to large? TODO(miek).
+	if soa && m.Rcode == dns.RcodeSuccess {
+		return NoData, opt
+	}
+	if soa && m.Rcode == dns.RcodeNameError {
+		return NameError, opt
+	}
+
+	if ns > 0 && ns == len(m.Ns) && m.Rcode == dns.RcodeSuccess {
+		return Delegation, opt
+	}
+
+	if m.Rcode == dns.RcodeSuccess {
+		return Success, opt
+	}
+
+	return OtherError, opt
+}