Add contrib/statfs tool

This should help debugging

https://github.com/rfjakob/gocryptfs/issues/375 and
https://github.com/rfjakob/gocryptfs/issues/274 ,

as MacOS does not have "stat -f".

contrib/statfs/.gitignore

@@ -0,0 +1 @@
+/statfs

contrib/statfs/statfs.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"os"
+	"syscall"
+)
+
+const (
+	myName = "statfs"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, "Usage: %s PATH\n", myName)
+		fmt.Fprintf(os.Stderr, "Dump the statfs information for PATH to the console, JSON format.\n")
+		os.Exit(1)
+	}
+	flag.Parse()
+	if flag.NArg() != 1 {
+		flag.Usage()
+	}
+	path := flag.Arg(0)
+	var st syscall.Statfs_t
+	err := syscall.Statfs(path, &st)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "statfs syscall returned error: %v\n", err)
+		os.Exit(2)
+	}
+	jsn, _ := json.MarshalIndent(st, "", "\t")
+	fmt.Println(string(jsn))
+}