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/proxy/fuzz.go

@@ -0,0 +1,19 @@
+package proxy
+
+import (
+	"github.com/coredns/coredns/plugin/pkg/fuzz"
+
+	"github.com/mholt/caddy"
+)
+
+// Fuzz fuzzes proxy.
+func Fuzz(data []byte) int {
+	c := caddy.NewTestController("dns", "proxy . 8.8.8.8:53")
+	up, err := NewStaticUpstreams(&c.Dispenser)
+	if err != nil {
+		return 0
+	}
+	p := &Proxy{Upstreams: &up}
+
+	return fuzz.Do(p, data)
+}