core: add missing trimzone files

*ugh*: forgot to add these files. This add the dnsutil.TrimZone
function.

middleware/pkg/dnsutil/zone.go

+package dnsutil
+
+import (
+	"errors"
+
+	"github.com/miekg/dns"
+)
+
+// TrimZone removes the zone component from q. It returns the trimmed
+// name or an error is zone is longer then qname. The trimmed name will be returned
+// without a trailing dot.
+func TrimZone(q string, z string) (string, error) {
+	zl := dns.CountLabel(z)
+	i, ok := dns.PrevLabel(q, zl)
+	if ok || i-1 < 0 {
+		return "", errors.New("trimzone: overshot qname: " + q + "for zone " + z)
+	}
+	// This includes the '.', remove on return
+	return q[:i-1], nil
+}

middleware/pkg/dnsutil/zone_test.go

+package dnsutil
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestTrimZone(t *testing.T) {
+	tests := []struct {
+		qname    string
+		zone     string
+		expected string
+		err      error
+	}{
+		{"a.example.org", "example.org", "a", nil},
+		{"a.b.example.org", "example.org", "a.b", nil},
+		{"b.", ".", "b", nil},
+		{"example.org", "example.org", "", errors.New("should err")},
+		{"org", "example.org", "", errors.New("should err")},
+	}
+
+	for i, tc := range tests {
+		got, err := TrimZone(dns.Fqdn(tc.qname), dns.Fqdn(tc.zone))
+		if tc.err != nil && err == nil {
+			t.Errorf("Test %d, expected error got nil")
+			continue
+		}
+		if tc.err == nil && err != nil {
+			t.Errorf("Test %d, expected no error got %v", i, err)
+			continue
+		}
+		if got != tc.expected {
+			t.Errorf("Test %d, expected %s, got %s", i, tc.expected, got)
+			continue
+		}
+	}
+}