Add contrib/getdents-debug/readdirnames

Another way to repro the problem in
https://github.com/rfjakob/gocryptfs/issues/483

contrib/getdents-debug/readdirnames/.gitignore

@@ -0,0 +1 @@
+/readdirnames

contrib/getdents-debug/readdirnames/readdirnames.go

@@ -0,0 +1,56 @@
+/*
+Small tool to try to debug unix.Getdents problems on CIFS mounts
+( https://github.com/rfjakob/gocryptfs/issues/483 )
+
+Example output:
+
+$ while sleep 1 ; do ./readdirnames /mnt/synology/public/tmp/g1 ; done
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=868, err=readdirent: no such file or directory
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+2020/05/24 23:50:39 os.Open returned err=open /mnt/synology/public/tmp/g1: interrupted system call
+Readdirnames: len=1001, err=<nil>
+Readdirnames: len=1001, err=<nil>
+*/
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+)
+
+const (
+	myName = "readdirnames"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, "Usage: %s PATH\n", myName)
+		fmt.Fprintf(os.Stderr, "Run os.File.Readdirnames on PATH\n")
+		os.Exit(1)
+	}
+	flag.Parse()
+	if flag.NArg() != 1 {
+		flag.Usage()
+	}
+	path := flag.Arg(0)
+
+	f, err := os.Open(path)
+	if err != nil {
+		log.Fatalf("os.Open returned err=%v", err)
+	}
+
+	names, err := f.Readdirnames(0)
+	fmt.Printf("Readdirnames: len=%d, err=%v\n", len(names), err)
+}