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/file/dname.go

@@ -0,0 +1,44 @@
+package file
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+
+	"github.com/miekg/dns"
+)
+
+// substituteDNAME performs the DNAME substitution defined by RFC 6672,
+// assuming the QTYPE of the query is not DNAME. It returns an empty
+// string if there is no match.
+func substituteDNAME(qname, owner, target string) string {
+	if dns.IsSubDomain(owner, qname) && qname != owner {
+		labels := dns.SplitDomainName(qname)
+		labels = append(labels[0:len(labels)-dns.CountLabel(owner)], dns.SplitDomainName(target)...)
+
+		return dnsutil.Join(labels)
+	}
+
+	return ""
+}
+
+// synthesizeCNAME returns a CNAME RR pointing to the resulting name of
+// the DNAME substitution. The owner name of the CNAME is the QNAME of
+// the query and the TTL is the same as the corresponding DNAME RR.
+//
+// It returns nil if the DNAME substitution has no match.
+func synthesizeCNAME(qname string, d *dns.DNAME) *dns.CNAME {
+	target := substituteDNAME(qname, d.Header().Name, d.Target)
+	if target == "" {
+		return nil
+	}
+
+	r := new(dns.CNAME)
+	r.Hdr = dns.RR_Header{
+		Name:   qname,
+		Rrtype: dns.TypeCNAME,
+		Class:  dns.ClassINET,
+		Ttl:    d.Header().Ttl,
+	}
+	r.Target = target
+
+	return r
+}