Use a default callback if the user omitted one. Avoids errors like the one
below:
fs.js:777
if (err) return callback(err);
^
TypeError: object is not a function
at fs.appendFile (fs.js:777:21)
at Object.oncomplete (fs.js:297:15)
This commit fixes the behavior of fs.lchmod(), fs.lchown() and fs.readFile()
when the callback is omitted. Before, they silently swallowed errors.
Fixes#4352.
a80434736b
