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/rewrite/class.go

@@ -0,0 +1,35 @@
+package rewrite
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+type classRule struct {
+	fromClass, toClass uint16
+}
+
+func newClassRule(fromS, toS string) (Rule, error) {
+	var from, to uint16
+	var ok bool
+	if from, ok = dns.StringToClass[strings.ToUpper(fromS)]; !ok {
+		return nil, fmt.Errorf("invalid class %q", strings.ToUpper(fromS))
+	}
+	if to, ok = dns.StringToClass[strings.ToUpper(toS)]; !ok {
+		return nil, fmt.Errorf("invalid class %q", strings.ToUpper(toS))
+	}
+	return &classRule{fromClass: from, toClass: to}, nil
+}
+
+// Rewrite rewrites the the current request.
+func (rule *classRule) Rewrite(w dns.ResponseWriter, r *dns.Msg) Result {
+	if rule.fromClass > 0 && rule.toClass > 0 {
+		if r.Question[0].Qclass == rule.fromClass {
+			r.Question[0].Qclass = rule.toClass
+			return RewriteDone
+		}
+	}
+	return RewriteIgnored
+}