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/test/responsewriter.go

@@ -0,0 +1,61 @@
+package test
+
+import (
+	"net"
+
+	"github.com/miekg/dns"
+)
+
+// ResponseWriter is useful for writing tests. It uses some fixed values for the client. The
+// remote will always be 10.240.0.1 and port 40212. The local address is always 127.0.0.1 and
+// port 53.
+type ResponseWriter struct{}
+
+// LocalAddr returns the local address, always 127.0.0.1:53 (UDP).
+func (t *ResponseWriter) LocalAddr() net.Addr {
+	ip := net.ParseIP("127.0.0.1")
+	port := 53
+	return &net.UDPAddr{IP: ip, Port: port, Zone: ""}
+}
+
+// RemoteAddr returns the remote address, always 10.240.0.1:40212 (UDP).
+func (t *ResponseWriter) RemoteAddr() net.Addr {
+	ip := net.ParseIP("10.240.0.1")
+	port := 40212
+	return &net.UDPAddr{IP: ip, Port: port, Zone: ""}
+}
+
+// WriteMsg implement dns.ResponseWriter interface.
+func (t *ResponseWriter) WriteMsg(m *dns.Msg) error { return nil }
+
+// Write implement dns.ResponseWriter interface.
+func (t *ResponseWriter) Write(buf []byte) (int, error) { return len(buf), nil }
+
+// Close implement dns.ResponseWriter interface.
+func (t *ResponseWriter) Close() error { return nil }
+
+// TsigStatus implement dns.ResponseWriter interface.
+func (t *ResponseWriter) TsigStatus() error { return nil }
+
+// TsigTimersOnly implement dns.ResponseWriter interface.
+func (t *ResponseWriter) TsigTimersOnly(bool) { return }
+
+// Hijack implement dns.ResponseWriter interface.
+func (t *ResponseWriter) Hijack() { return }
+
+// RepsponseWrite6 returns fixed client and remote address in IPv6.  The remote
+// address is always fe80::42:ff:feca:4c65 and port 40212. The local address
+// is always ::1 and port 53.
+type ResponseWriter6 struct {
+	ResponseWriter
+}
+
+// LocalAddr returns the local address, always ::1, port 53 (UDP).
+func (t *ResponseWriter6) LocalAddr() net.Addr {
+	return &net.UDPAddr{IP: net.ParseIP("::1"), Port: 53, Zone: ""}
+}
+
+// RemoteAddr returns the remote address, always fe80::42:ff:feca:4c65 port 40212 (UDP).
+func (t *ResponseWriter6) RemoteAddr() net.Addr {
+	return &net.UDPAddr{IP: net.ParseIP("fe80::42:ff:feca:4c65"), Port: 40212, Zone: ""}
+}