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/kubernetes/apiproxy.go

@@ -0,0 +1,76 @@
+package kubernetes
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+
+	"github.com/coredns/coredns/plugin/pkg/healthcheck"
+)
+
+type proxyHandler struct {
+	healthcheck.HealthCheck
+}
+
+type apiProxy struct {
+	http.Server
+	listener net.Listener
+	handler  proxyHandler
+}
+
+func (p *proxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	upstream := p.Select()
+	network := "tcp"
+	if upstream.Network != "" {
+		network = upstream.Network
+	}
+	address := upstream.Name
+	d, err := net.Dial(network, address)
+	if err != nil {
+		log.Printf("[ERROR] Unable to establish connection to upstream %s://%s: %s", network, address, err)
+		http.Error(w, fmt.Sprintf("Unable to establish connection to upstream %s://%s: %s", network, address, err), 500)
+		return
+	}
+	hj, ok := w.(http.Hijacker)
+	if !ok {
+		log.Printf("[ERROR] Unable to establish connection: no hijacker")
+		http.Error(w, "Unable to establish connection: no hijacker", 500)
+		return
+	}
+	nc, _, err := hj.Hijack()
+	if err != nil {
+		log.Printf("[ERROR] Unable to hijack connection: %s", err)
+		http.Error(w, fmt.Sprintf("Unable to hijack connection: %s", err), 500)
+		return
+	}
+	defer nc.Close()
+	defer d.Close()
+
+	err = r.Write(d)
+	if err != nil {
+		log.Printf("[ERROR] Unable to copy connection to upstream %s://%s: %s", network, address, err)
+		http.Error(w, fmt.Sprintf("Unable to copy connection to upstream %s://%s: %s", network, address, err), 500)
+		return
+	}
+
+	errChan := make(chan error, 2)
+	cp := func(dst io.Writer, src io.Reader) {
+		_, err := io.Copy(dst, src)
+		errChan <- err
+	}
+	go cp(d, nc)
+	go cp(nc, d)
+	<-errChan
+}
+
+func (p *apiProxy) Run() {
+	p.handler.Start()
+	p.Serve(p.listener)
+}
+
+func (p *apiProxy) Stop() {
+	p.handler.Stop()
+	p.listener.Close()
+}