fs: move makeCallback and maybeCallback to util

`makeCallback()`, `maybeCallback()`, and `rethrow()` declared in **fs** can also be used in other places.

Related to [#6225](https://github.com/joyent/node/issues/6525)

I don't know why the tests are failing but apparently the failures are not related to my changes.  

PS:
I already signed the CLA.