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

directives_generate.go

@@ -17,7 +17,7 @@ func main() {
 	mi := make(map[string]string, 0)
 	md := make(map[int]string, 0)
 
-	file, err := os.Open(middlewareFile)
+	file, err := os.Open(pluginFile)
 	fatalIfErr(err)
 
 	defer file.Close()
@@ -41,14 +41,14 @@ func main() {
 			log.Fatalf("Duplicate priority '%d', slot already taken by %q", priority, v)
 		}
 		md[priority] = items[1]
-		mi[items[1]] = middlewarePath + items[2] // Default, unless overridden by 3rd arg
+		mi[items[1]] = pluginPath + items[2] // Default, unless overridden by 3rd arg
 
-		if _, err := os.Stat(middlewareFSPath + items[2]); err != nil { // External package has been given
+		if _, err := os.Stat(pluginFSPath + items[2]); err != nil { // External package has been given
 			mi[items[1]] = items[2]
 		}
 	}
 
-	genImports("core/zmiddleware.go", "core", mi)
+	genImports("core/zplugin.go", "core", mi)
 	genDirectives("core/dnsserver/zdirectives.go", "dnsserver", md)
 }
 
@@ -59,7 +59,7 @@ func genImports(file, pack string, mi map[string]string) {
 		outs += "\n"
 	}
 
-	outs += "// Include all middleware.\n"
+	outs += "// Include all plugin.\n"
 	for _, v := range mi {
 		outs += `_ "` + v + `"` + "\n"
 	}
@@ -79,10 +79,10 @@ func genDirectives(file, pack string, md map[int]string) {
 // Directives are registered in the order they should be
 // executed.
 //
-// Ordering is VERY important. Every middleware will
-// feel the effects of all other middleware below
+// Ordering is VERY important. Every plugin will
+// feel the effects of all other plugin below
 // (after) them during a request, but they must not
-// care what middleware above them are doing.
+// care what plugin above them are doing.
 
 var directives = []string{
 `
@@ -113,8 +113,8 @@ func fatalIfErr(err error) {
 }
 
 const (
-	middlewarePath   = "github.com/coredns/coredns/middleware/"
-	middlewareFile   = "middleware.cfg"
-	middlewareFSPath = "middleware/" // Where the middleware packages are located on the file system
-	header           = "// generated by directives_generate.go; DO NOT EDIT\n\n"
+	pluginPath   = "github.com/coredns/coredns/plugin/"
+	pluginFile   = "plugin.cfg"
+	pluginFSPath = "plugin/" // Where the plugins are located on the file system
+	header       = "// generated by directives_generate.go; DO NOT EDIT\n\n"
 )