Adding fileutils.DevNull convenience

The intended use case is with os.Exec.Cmd - it is often convenient to initialize a cmd struct in the following way:

```go
&exec.Cmd{
  //...
  Stdout: os.Stdout,
  Stderr: os.Stderr,
  //...
}
```

However, an attempt to use `os.DevNull` in the same context does not work, as that is actually just the string `"/dev/null"`.  Adding these `io.Writer` handles to `fileutils` will help remove some confusion by making all three accessible in the same package.  Also adding `os.Stdin` for the sake of completion