PR26125: debuginfod client cache - rmdir harder

PR26125: debuginfod client cache - rmdir harder

With PR25365, we accidentally lost the ability to rmdir client-cache
directories corresponding to buildids.  Bring this back, with some
attention to a possible race between a client doing cleanup and
another client doing lookups at the same time.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>