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/file/closest_test.go

@@ -0,0 +1,38 @@
+package file
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestClosestEncloser(t *testing.T) {
+	z, err := Parse(strings.NewReader(dbMiekNL), testzone, "stdin", 0)
+	if err != nil {
+		t.Fatalf("expect no error when reading zone, got %q", err)
+	}
+
+	tests := []struct {
+		in, out string
+	}{
+		{"miek.nl.", "miek.nl."},
+		{"www.miek.nl.", "www.miek.nl."},
+
+		{"blaat.miek.nl.", "miek.nl."},
+		{"blaat.www.miek.nl.", "www.miek.nl."},
+		{"www.blaat.miek.nl.", "miek.nl."},
+		{"blaat.a.miek.nl.", "a.miek.nl."},
+	}
+
+	for _, tc := range tests {
+		ce, _ := z.ClosestEncloser(tc.in)
+		if ce == nil {
+			if z.origin != tc.out {
+				t.Errorf("Expected ce to be %s for %s, got %s", tc.out, tc.in, ce.Name())
+			}
+			continue
+		}
+		if ce.Name() != tc.out {
+			t.Errorf("Expected ce to be %s for %s, got %s", tc.out, tc.in, ce.Name())
+		}
+	}
+}