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.

Makefile.fuzz

@@ -0,0 +1,33 @@
+# Makefile for fuzzing
+#
+# Use go-fuzz and needs the tools installed. For each fuzz.go in a plugin's directory
+# you can start the fuzzing with: make -f Makefile.fuzz <plugin>
+# e.g.
+#
+# 	make -f Makefile.fuzz proxy
+# 
+# Each plugin that wants to join the fuzzing fray only needs to add a fuzz.go that calls
+# the plugins's ServeDNS and used the plugin/pkg/fuzz for the Do function.
+#
+# Installing go-fuzz
+#$ go get github.com/dvyukov/go-fuzz/go-fuzz
+#$ go get github.com/dvyukov/go-fuzz/go-fuzz-build
+
+REPO:="github.com/coredns/coredns/plugin"
+
+FUZZ:=$(dir $(wildcard plugin/*/fuzz.go)) # plugin/cache/
+PLUGINS:=$(foreach f,$(FUZZ),$(subst plugin, ,$(f:/=))) # > /cache
+PLUGINS:=$(foreach f,$(PLUGINS),$(subst /, ,$(f))) # > cache
+
+.PHONY: echo
+echo:
+	@echo fuzz targets: $(PLUGINS)
+
+.PHONY: $(PLUGINS)
+$(PLUGINS): echo
+	go-fuzz-build $(REPO)/$(@)
+	go-fuzz -bin=./$(@)-fuzz.zip -workdir=fuzz/$(@)
+
+.PHONY: clean
+clean:
+	rm *-fuzz.zip