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/edns/edns.go

@@ -0,0 +1,45 @@
+package edns
+
+import (
+	"errors"
+
+	"github.com/miekg/dns"
+)
+
+// Version checks the EDNS version in the request. If error
+// is nil everything is OK and we can invoke the middleware. If non-nil, the
+// returned Msg is valid to be returned to the client (and should). For some
+// reason this response should not contain a question RR in the question section.
+func Version(req *dns.Msg) (*dns.Msg, error) {
+	opt := req.IsEdns0()
+	if opt == nil {
+		return nil, nil
+	}
+	if opt.Version() == 0 {
+		return nil, nil
+	}
+	m := new(dns.Msg)
+	m.SetReply(req)
+	// zero out question section, wtf.
+	m.Question = nil
+
+	o := new(dns.OPT)
+	o.Hdr.Name = "."
+	o.Hdr.Rrtype = dns.TypeOPT
+	o.SetVersion(0)
+	o.SetExtendedRcode(dns.RcodeBadVers)
+	m.Extra = []dns.RR{o}
+
+	return m, errors.New("EDNS0 BADVERS")
+}
+
+// Size returns a normalized size based on proto.
+func Size(proto string, size int) int {
+	if proto == "tcp" {
+		return dns.MaxMsgSize
+	}
+	if size < dns.MinMsgSize {
+		return dns.MinMsgSize
+	}
+	return size
+}

middleware/pkg/edns/edns_test.go

@@ -0,0 +1,37 @@
+package edns
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestVersion(t *testing.T) {
+	m := ednsMsg()
+	m.Extra[0].(*dns.OPT).SetVersion(2)
+
+	_, err := Version(m)
+	if err == nil {
+		t.Errorf("expected wrong version, but got OK")
+	}
+}
+
+func TestVersionNoEdns(t *testing.T) {
+	m := ednsMsg()
+	m.Extra = nil
+
+	_, err := Version(m)
+	if err != nil {
+		t.Errorf("expected no error, but got one: %s", err)
+	}
+}
+
+func ednsMsg() *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+	o := new(dns.OPT)
+	o.Hdr.Name = "."
+	o.Hdr.Rrtype = dns.TypeOPT
+	m.Extra = append(m.Extra, o)
+	return m
+}