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/health/health.go

@@ -0,0 +1,69 @@
+// Package health implements an HTTP handler that responds to health checks.
+package health
+
+import (
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"sync"
+)
+
+var once sync.Once
+
+type health struct {
+	Addr string
+
+	ln  net.Listener
+	mux *http.ServeMux
+
+	// A slice of Healthers that the health plugin will poll every second for their health status.
+	h []Healther
+	sync.RWMutex
+	ok bool // ok is the global boolean indicating an all healthy plugin stack
+}
+
+func (h *health) Startup() error {
+	if h.Addr == "" {
+		h.Addr = defAddr
+	}
+
+	once.Do(func() {
+		ln, err := net.Listen("tcp", h.Addr)
+		if err != nil {
+			log.Printf("[ERROR] Failed to start health handler: %s", err)
+			return
+		}
+
+		h.ln = ln
+
+		h.mux = http.NewServeMux()
+
+		h.mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
+			if h.Ok() {
+				w.WriteHeader(http.StatusOK)
+				io.WriteString(w, ok)
+				return
+			}
+			w.WriteHeader(http.StatusServiceUnavailable)
+		})
+
+		go func() {
+			http.Serve(h.ln, h.mux)
+		}()
+	})
+	return nil
+}
+
+func (h *health) Shutdown() error {
+	if h.ln != nil {
+		return h.ln.Close()
+	}
+	return nil
+}
+
+const (
+	ok      = "OK"
+	defAddr = ":8080"
+	path    = "/health"
+)