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/cache/fuzz.go

@@ -0,0 +1,14 @@
+package cache
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/fuzz"
+)
+
+// Fuzz fuzzes cache.
+func Fuzz(data []byte) int {
+	c := &Cache{pcap: defaultCap, ncap: defaultCap, pttl: maxTTL, nttl: maxNTTL, prefetch: 0, duration: 1 * time.Minute}
+
+	return fuzz.Do(c, data)
+}