apr_atomic stuff... desireable to return values from more functions

[Jeff Trawick]

This was discussed previously in the thread "request for comments: new atomic 
API" but the thread died out.

I'm not about to jump on this because of lack of mojo to get the various 
platform-specific bits to work, but I'm willing to teach the z/OS code to 
conform if Greg doesn't beat me to it.  (Atomics are busted currently on z/OS 
because we DON'T return a value from add32().)