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/rewrite.go

@@ -0,0 +1,86 @@
+package rewrite
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/miekg/dns"
+
+	"golang.org/x/net/context"
+)
+
+// Result is the result of a rewrite
+type Result int
+
+const (
+	// RewriteIgnored is returned when rewrite is not done on request.
+	RewriteIgnored Result = iota
+	// RewriteDone is returned when rewrite is done on request.
+	RewriteDone
+	// RewriteStatus is returned when rewrite is not needed and status code should be set
+	// for the request.
+	RewriteStatus
+)
+
+// Rewrite is plugin to rewrite requests internally before being handled.
+type Rewrite struct {
+	Next     plugin.Handler
+	Rules    []Rule
+	noRevert bool
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (rw Rewrite) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	wr := NewResponseReverter(w, r)
+	for _, rule := range rw.Rules {
+		switch result := rule.Rewrite(w, r); result {
+		case RewriteDone:
+			if rw.noRevert {
+				return plugin.NextOrFailure(rw.Name(), rw.Next, ctx, w, r)
+			}
+			return plugin.NextOrFailure(rw.Name(), rw.Next, ctx, wr, r)
+		case RewriteIgnored:
+			break
+		case RewriteStatus:
+			// only valid for complex rules.
+			// if cRule, ok := rule.(*ComplexRule); ok && cRule.Status != 0 {
+			// return cRule.Status, nil
+			// }
+		}
+	}
+	return plugin.NextOrFailure(rw.Name(), rw.Next, ctx, w, r)
+}
+
+// Name implements the Handler interface.
+func (rw Rewrite) Name() string { return "rewrite" }
+
+// Rule describes a rewrite rule.
+type Rule interface {
+	// Rewrite rewrites the current request.
+	Rewrite(dns.ResponseWriter, *dns.Msg) Result
+}
+
+func newRule(args ...string) (Rule, error) {
+	if len(args) == 0 {
+		return nil, fmt.Errorf("no rule type specified for rewrite")
+	}
+
+	ruleType := strings.ToLower(args[0])
+	if ruleType != "edns0" && len(args) != 3 {
+		return nil, fmt.Errorf("%s rules must have exactly two arguments", ruleType)
+	}
+	switch ruleType {
+	case "name":
+		return newNameRule(args[1], args[2])
+	case "class":
+		return newClassRule(args[1], args[2])
+	case "type":
+		return newTypeRule(args[1], args[2])
+	case "edns0":
+		return newEdns0Rule(args[1:]...)
+	default:
+		return nil, fmt.Errorf("invalid rule type %q", args[0])
+	}
+}