Fixed max-stale check. Entities not exceeding max-stale were marked as stale.

Fixed max-stale check. Entities not exceeding max-stale were marked as stale.

Since the fixed check is performed for entities already suspected of being
stale by refreshCheck(), it is difficult to describe exactly which entities
were affected by the bug. A rough description would be "entities which would
otherwise qualify for a FRESH_OVERRIDE_EXPIRES or FRESH_OVERRIDE_LASTMOD
exceptions located below the fixed check.

Other concerns about staleness checks have been discussed on squid-dev's
"max_stale broken?" email thread.