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/pkg/edns/edns_test.go

@@ -0,0 +1,37 @@
+package edns
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestVersion(t *testing.T) {
+	m := ednsMsg()
+	m.Extra[0].(*dns.OPT).SetVersion(2)
+
+	_, err := Version(m)
+	if err == nil {
+		t.Errorf("expected wrong version, but got OK")
+	}
+}
+
+func TestVersionNoEdns(t *testing.T) {
+	m := ednsMsg()
+	m.Extra = nil
+
+	_, err := Version(m)
+	if err != nil {
+		t.Errorf("expected no error, but got one: %s", err)
+	}
+}
+
+func ednsMsg() *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+	o := new(dns.OPT)
+	o.Hdr.Name = "."
+	o.Hdr.Rrtype = dns.TypeOPT
+	m.Extra = append(m.Extra, o)
+	return m
+}