[PATCH] external ACL caching update

Small optimization to the external ACL caching behaviour. With bigger than
itself effects.

Admin configure ttl=0 and/or negative_ttl=0 to prevent Squid storing the
ACL lookup results. The problem is that results still get cached and
re-used for the grace= period. IMO this grace period should only occur for
results which are allowed to be re-used.

The attached patch changes the external ACL caching slightly to actually
not store the results when their matching TTL is zero.

Side effects of this (on systems with ttl <= 0 or negative_ttl <= 0) are:

 less memory usage

 expected non-caching works

 FAST category ACL tests not able to be used on non-cached results

 less processing cycles on result hash insertion
  * traded for potential cycles on hash lookups

Amos