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/response/classify.go

@@ -0,0 +1,61 @@
+package response
+
+import "fmt"
+
+// Class holds sets of Types
+type Class int
+
+const (
+	// All is a meta class encompassing all the classes.
+	All Class = iota
+	// Success is a class for a successful response.
+	Success
+	// Denial is a class for denying existence (NXDOMAIN, or a nodata: type does not exist)
+	Denial
+	// Error is a class for errors, right now defined as not Success and not Denial
+	Error
+)
+
+func (c Class) String() string {
+	switch c {
+	case All:
+		return "all"
+	case Success:
+		return "success"
+	case Denial:
+		return "denial"
+	case Error:
+		return "error"
+	}
+	return ""
+}
+
+// ClassFromString returns the class from the string s. If not class matches
+// the All class and an error are returned
+func ClassFromString(s string) (Class, error) {
+	switch s {
+	case "all":
+		return All, nil
+	case "success":
+		return Success, nil
+	case "denial":
+		return Denial, nil
+	case "error":
+		return Error, nil
+	}
+	return All, fmt.Errorf("invalid Class: %s", s)
+}
+
+// Classify classifies the Type t, it returns its Class.
+func Classify(t Type) Class {
+	switch t {
+	case NoError, Delegation:
+		return Success
+	case NameError, NoData:
+		return Denial
+	case OtherError:
+		fallthrough
+	default:
+		return Error
+	}
+}