Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/pkg/dnsutil/zone_test.go

@@ -0,0 +1,39 @@
+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", i)
+			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
+		}
+	}
+}