Support for os_retain() and os_release()

[theiostream]

In more recent libdispatch releases, os/object.h comes with os_retain() and os_release() functions, as can be seen in https://opensource.apple.com/source/libdispatch/libdispatch-685/os/object.h. It'd be nice to have them in this fork for compatibility purposes with newer applications.

Is there any issue with just merging them in this repository? (I can provide a pull request for that, but not before knowing if there would be any negative implication or if there is already an ongoing effort to rebase on top of Apple's changes to libdispatch which would cover this.)