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/zone.go

@@ -0,0 +1,20 @@
+package dnsutil
+
+import (
+	"errors"
+
+	"github.com/miekg/dns"
+)
+
+// TrimZone removes the zone component from q. It returns the trimmed
+// name or an error is zone is longer then qname. The trimmed name will be returned
+// without a trailing dot.
+func TrimZone(q string, z string) (string, error) {
+	zl := dns.CountLabel(z)
+	i, ok := dns.PrevLabel(q, zl)
+	if ok || i-1 < 0 {
+		return "", errors.New("trimzone: overshot qname: " + q + "for zone " + z)
+	}
+	// This includes the '.', remove on return
+	return q[:i-1], nil
+}