Rename awaitEvent in select and win32 I/O managers

These are now just called from IOManager.c and are the per-I/O manager
backend impls (whereas previously awaitEvent was the entry point).

Follow the new naming convention in the IOManager.{h,c} of
awaitCompletedTimeoutsOrIO, with the I/O manager's name as a suffix:
so awaitCompletedTimeoutsOrIO{Select,Win32}.