One-shot semantics in GHC event manager

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

Ben,

I have some comments and questions about your code:

C: registerFd' is exported. So, it should have document.

Q: Since registerFd uses OneShot and threadWait uses registerFd, basic
   IO functions use OneShot by default. No changes from GHC 7.8.3. Do
   I understand correctly?

Q: dbus library will use registerFd' to specify MultiShot, right?

--Kazu