Add fuzzing infrastructure (#1118)

Fix file/fuzz.go build and docs in Makefile.fuzz
Each plugin can add a fuzz.go to join the fuzzing craze.
pkg/fuzz/do.go could be made a lot smarter, but is probably good enough
for starters.

$ make -f Makefile.fuzz <plugin>

will build with go-fuzz-build and then execute a go-fuzz run. Each
plugin's fuzz run uses a per-plugin directory to store the fuzz data.

plugin/pkg/fuzz/do.go

@@ -0,0 +1,25 @@
+package fuzz
+
+import (
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Do will fuzz p - used by gofuzz. See Maefile.fuzz for comments and context.
+func Do(p plugin.Handler, data []byte) int {
+	ctx := context.TODO()
+	ret := 1
+	r := new(dns.Msg)
+	if err := r.Unpack(data); err != nil {
+		ret = 0
+	}
+
+	if _, err := p.ServeDNS(ctx, &test.ResponseWriter{}, r); err != nil {
+		ret = 1
+	}
+
+	return ret
+}