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/join.go

@@ -0,0 +1,19 @@
+package dnsutil
+
+import (
+	"strings"
+
+	"github.com/miekg/dns"
+)
+
+// Join joins labels to form a fully qualified domain name. If the last label is
+// the root label it is ignored. Not other syntax checks are performed.
+func Join(labels []string) string {
+	ll := len(labels)
+	if labels[ll-1] == "." {
+		s := strings.Join(labels[:ll-1], ".")
+		return dns.Fqdn(s)
+	}
+	s := strings.Join(labels, ".")
+	return dns.Fqdn(s)
+}