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/dnstap/out/socket_test.go

@@ -0,0 +1,94 @@
+package out
+
+import (
+	"net"
+	"testing"
+
+	fs "github.com/farsightsec/golang-framestream"
+)
+
+func acceptOne(t *testing.T, l net.Listener) {
+	server, err := l.Accept()
+	if err != nil {
+		t.Fatalf("server accept: %s", err)
+		return
+	}
+
+	dec, err := fs.NewDecoder(server, &fs.DecoderOptions{
+		ContentType:   []byte("protobuf:dnstap.Dnstap"),
+		Bidirectional: true,
+	})
+	if err != nil {
+		t.Fatalf("server decoder: %s", err)
+		return
+	}
+
+	if _, err := dec.Decode(); err != nil {
+		t.Errorf("server decode: %s", err)
+	}
+
+	if err := server.Close(); err != nil {
+		t.Error(err)
+	}
+}
+func sendOne(socket *Socket) error {
+	if _, err := socket.Write([]byte("frame")); err != nil {
+		return err
+	}
+	if err := socket.enc.Flush(); err != nil {
+		// Would happen during Write in real life.
+		socket.conn.Close()
+		socket.err = err
+		return err
+	}
+	return nil
+}
+func TestSocket(t *testing.T) {
+	socket, err := NewSocket("dnstap.sock")
+	if err == nil {
+		t.Fatal("new socket: not listening but no error")
+		return
+	}
+
+	if err := sendOne(socket); err == nil {
+		t.Fatal("not listening but no error")
+		return
+	}
+
+	l, err := net.Listen("unix", "dnstap.sock")
+	if err != nil {
+		t.Fatal(err)
+		return
+	}
+
+	wait := make(chan bool)
+	go func() {
+		acceptOne(t, l)
+		wait <- true
+	}()
+
+	if err := sendOne(socket); err != nil {
+		t.Fatalf("send one: %s", err)
+		return
+	}
+
+	<-wait
+	if err := sendOne(socket); err == nil {
+		panic("must fail")
+	}
+
+	go func() {
+		acceptOne(t, l)
+		wait <- true
+	}()
+
+	if err := sendOne(socket); err != nil {
+		t.Fatalf("send one: %s", err)
+		return
+	}
+
+	<-wait
+	if err := l.Close(); err != nil {
+		t.Error(err)
+	}
+}