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/chaos/chaos.go

@@ -0,0 +1,62 @@
+// Package chaos implements a plugin that answer to 'CH version.bind TXT' type queries.
+package chaos
+
+import (
+	"os"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Chaos allows CoreDNS to reply to CH TXT queries and return author or
+// version information.
+type Chaos struct {
+	Next    plugin.Handler
+	Version string
+	Authors map[string]bool
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (c Chaos) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+	if state.QClass() != dns.ClassCHAOS || state.QType() != dns.TypeTXT {
+		return plugin.NextOrFailure(c.Name(), c.Next, ctx, w, r)
+	}
+
+	m := new(dns.Msg)
+	m.SetReply(r)
+
+	hdr := dns.RR_Header{Name: state.QName(), Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
+	switch state.Name() {
+	default:
+		return c.Next.ServeDNS(ctx, w, r)
+	case "authors.bind.":
+		for a := range c.Authors {
+			m.Answer = append(m.Answer, &dns.TXT{Hdr: hdr, Txt: []string{trim(a)}})
+		}
+	case "version.bind.", "version.server.":
+		m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{trim(c.Version)}}}
+	case "hostname.bind.", "id.server.":
+		hostname, err := os.Hostname()
+		if err != nil {
+			hostname = "localhost"
+		}
+		m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{trim(hostname)}}}
+	}
+	state.SizeAndDo(m)
+	w.WriteMsg(m)
+	return 0, nil
+}
+
+// Name implements the Handler interface.
+func (c Chaos) Name() string { return "chaos" }
+
+func trim(s string) string {
+	if len(s) < 256 {
+		return s
+	}
+	return s[:255]
+}