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/dnsutil/dedup.go

@@ -0,0 +1,12 @@
+package dnsutil
+
+import "github.com/miekg/dns"
+
+// Dedup de-duplicates a message.
+func Dedup(m *dns.Msg) *dns.Msg {
+	// TODO(miek): expensive!
+	m.Answer = dns.Dedup(m.Answer, nil)
+	m.Ns = dns.Dedup(m.Ns, nil)
+	m.Extra = dns.Dedup(m.Extra, nil)
+	return m
+}