fs: more misc optimizations

The main focus of this commit is to:

* Avoid options object copying where possible
* Use V8's object cloning only for default options objects where we
are in control of the object's configuration since the object clone
will also copy properties' configuration, frozen/sealed state, etc.
* Refactor writeFile*()/appendFile*() to reuse shared logic without
the redundant argument checking and to reuse FSReqWrap instances more

Author: mscdex