This makes several changes:

1. Allow path/filename to be passed in as a Buffer on fs methods
2. Add `options.encoding` to fs.readdir, fs.readdirSync, fs.readlink,
   fs.readlinkSync and fs.watch.
3. Documentation updates

For 1... it's now possible to do:

```js
fs.open(Buffer('/fs/foo/bar'), 'w+', (err, fd) => { });
```

For 2...
```js
fs.readdir('/fs/foo/bar', {encoding:'hex'}, (err,list) => { });

fs.readdir('/fs/foo/bar', {encoding:'buffer'}, (err, list) => { });
```

encoding can also be passed as a string

```js
fs.readdir('/fs/foo/bar', 'hex', (err,list) => { });
```

The default encoding is set to UTF8 so this addresses the
discrepency that existed previously between fs.readdir and
fs.watch handling filenames differently.

Fixes: #2088
Refs: #3519
PR-URL: #5616
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>