glib/gpoll W32: use WFSOE() instead of SleepEx()

WaitForSingleObjectEx() is supposed to be a more efficient sleep method.
It waits on the handle of the current process. That handle will be signaled once the process terminates, and since we're *inside* the process, it'll never happen (and if it does, we won't care anymore).
The use of an alertable wait ensures that we wake up when a completion routine wants to run.

https://bugzilla.gnome.org/show_bug.cgi?id=785468