Handle None from _downloadPackages (#949725)

YumBase._downloadPackages returns a list, unless it finds no updates, in
which case it returns None. Convert this return value to an empty list
so that the value can always be converted to a set.
Handle KeyboardInterrupt during download (#961955)

FedupDownloader.download_boot_images checks for IOErrors that are raised
as KeyboardInterrupt in older versions of yum.urlgrabber. This change
adds the other half of the conditional: if a KeyboardInterrupt is raised
and a newer urlgrabber is in use, raise the exception as-is.