Make callbacks use correct number of arguments

In the functions where 'fs' gives a callback of the form function(err),
mock-fs will give a callback of the form function(err, undefined). This
is not equivalent, and will mean using mock-fs functions in place like
some of async.js flow control functions will cause a different behaviour
than the native 'fs' functions.

See https://gist.github.com/bruse/5e5ba24bc5109090f75b for an example
of this inconsistent behavior.

This is one way of fixing it. If we are always sure that the functions that
are supposed to return data never returns undefined, we can make it a
lot shorter I suppose, but I chose to be explicit.

What do you think?