replace __sync_fetch_and_OP with __sync_OP_and_fetch

Hi,

I know that this patch is a bit irritating and it will not make any difference at all. But I wanted to contribute to such a cool software and found low hanging fruit :-)

Note that I changed op in ATOMIC_READ to OR. 

Also, it's quite possible that I'm stupid and you wrote this for very good and very smart reasons :-) If so, just disregard me (but explain, please :-))

P.S. return value of REFCNT_ATOMIC_INCREMENT/DECREMENT is never checked