glib-unix: fix handling of multiple signal source for the same signal
We can't reset the pending flag for a signal until we've traversed
the whole list, as the documentation clearly says that in case multiple
sources they all get invoked.
This is still racy if you get a signal after checking the flag
but before resetting it, but it was the same before. The correct
fix would be to use sigwait() or signalfd(), but that would mean
blocking all signals in all threads, which is not compatible
with existing applications.
https://bugzilla.gnome.org/show_bug.cgi?id=704322