PR29022: 000-permissions files cause problems for backups

PR29022: 000-permissions files cause problems for backups

000-permission files currently used for negative caching can cause
permission problems for some backup software and disk usage checkers.
Fix this by using empty files for negative caching instead.

Also use each empty file's mtime to determine the time since
last download attempt instead of the cache_miss_s file's mtime.

https://sourceware.org/bugzilla/show_bug.cgi?id=29022

Tested-by: Milian Wolff <mail@milianw.de>
Signed-off-by: Aaron Merey <amerey@redhat.com>